The amygdala is the core part of the limbic system (emotional processing) in our brains. It is the part we often refer to as our "lizard brain" because it produces our "primary emotions" such as fear, and sadness. It is also part of our "fight or flight" processing. Emotions that come from our amygdala are not under our control. In other words, if your amygdala is sending out messages that we are afraid, we cannot consciously tell it to shut up. What we CAN do however is use the higher processing power of our brains (pre-frontal cortex work) to decide how to respond to those primal emotions.
Anger and defensiveness are not primary emotions, they are chosen responses to primary emotions. When we are afraid of something, or sad (ashamed) about something, we often get upset as a result, because our brains start to "short circuit" as our fight or flight systems kick in as a response. The idea is to reprogram this process, to choose new, healthier responses to our fears and worries. Over time, how we respond to things that upset us can change. This process works particularly well for specific, reoccurring emotions to equally specific stimuli. In other words, if your spouse asks you to take the trash out every week, and every week you get pissed when they ask that, then that's easier to reprogram than something more general, such as reacting to upsetting stories on the evening news. But in both cases, we can change our responses by making conscious choices and telling our brains that this is "the new black".
There are two ways (or more) to approach this.
The first way is what I call "blind reprogramming". This method involves identifying a problem and choosing a new thought process. So given the above example, we may notice that we get upset every time we're asked to take the trash out. The way to fix this is to consciously be aware of our own anger and defensiveness, and when it happens again, we stop. Take a moment. Close your eyes and take a few deep breaths. Then start to give yourself instructions almost exactly as you would give a 5-year-old the same advice. "I am just being asked to take the trash out. I do this every week. Getting upset only makes it worse. From now on, instead of feeling upset, I am going to just get up and do it without complaint or further thought, and then when I'm done, it's over, and I won't think about it more". Every week, you go through this same process, making a conscious effort to put new thought patterns into your brain. This can work surprisingly well if you remain open to it. It can also work very quickly.
Part of the trick here is to not give in to the distractions that your brain will throw at you. It is quite possible that initially, your brain will fight even harder to make you upset. Don't fall for it. The best way to respond to this is to acknowledge it. Say to yourself, "Here I go again, I feel myself getting angry even though I'm trying not to. I am acknowledging this feeling and it is a valid feeling. I am not sweeping it under the rug. But not today, Satan. If I just take the trash out then the feeling goes away, so from now on, I choose to respond differently". Then take the damn trash out and get on with the rest of your life.
What scares most people off about this process is simply the fact that our brains will "fight the change" at first, and often, this can feel like our minds sending the same messages of anger and defensiveness so much and so fast that we barely have time to even start thinking something else. That's okay, that's normal, and natural. If your brain complains a thousand times, then correct it a thousand times. And don't expect perfection or success the first time, or second, or tenth. Each time, it gets a little better. Compare it to learning to play the violin. At first, you'll suck at it. A few months later however and you'll have some control you didn't before. A year later and you are playing a song. Keep practicing changes your thoughts, and over time, it will get easier to do, even with new challenges to overcome.
The second, more complete way to approach this is to do the same thing, but also to throw in understanding and compassion. This involves us digging to really understand not only THAT we are afraid or sad (which is resulting in our anger or defensiveness), but WHY we are feeling those emotions. Often, understanding the WHY of our emotions allows us to change our responses more easily and with even better results. Expanding on our example, we may realize that taking the trash out was something our parents made us do every week. We didn't ask for it, didn't want to do it, weren't given a choice, and then often, it became a point of argument or even shame. You might begin to realize that your parents used the trash as emotional blackmail (e.g. You want us to buy you a car, but you can't even do something (for us) as simple as taking the trash out without being screamed at, so why do you deserve anything from us?). Or they may simply point out that you never do it without being asked, which might make you feel ashamed for being "useless or unhelpful" in their eyes and yours. It's important to be really honest during this process. It can be so easy to just say, "Well, my Dad was an asshole about it, so it pisses me off" and leave it there, but that's not very self-reflective. Your dad didn't put a gun to your head and force you to be pissed off, you chose to be pissed off, so dig deeper. Whatever the answer is, it will be about YOU and only you, because only you feel your own emotions and only you choose how to respond to them.
It should go without saying that involving an IC in this process would be preferable, if possible. But you CAN do it on your own if needed.
This second process allows us to add understanding to our new programming. So in this case, we might make the following reasoning, "I'm an adult now. I'm not fighting with my parents anymore and so I don't have to go directly to anger when the trash has to go out. I can do it when I want to, how I want to. I'm not a kid anymore and don't want to think like one anymore either. Getting angry is stupid. It won't change anything and it doesn't make me feel good, in fact, it usually makes things worse. Screw that. It's my house, my life, and my trash, so I just need to "adult up" and do it and move on. From now on, the damn trash goes out every Friday at 5pm. I'll put it on my calendar and when it comes up, I'll just do it." And all of a sudden, the anger can be gone. You just start taking the trash out but skipping the whole part about getting upset. Pretty soon, you start applying this same thinking to other parts of your life too. You just pay the bills instead of complaining. You just let that asshole on the highway get in front of you without the middle finger and horn going off. You wash the dishes in the sink even though it's someone else's turn because you are there and can. You just start to CHOOSE your responses to your primary emotions, and in doing so...
Everything gets easier. You feel less stressed, less angry and upset all the time. Removing the defensiveness begins to feel like a ball and chain have been cut from your ankle and it is freeing to say the very least. You begin to realize that you have held yourself back in life because you let fear or sadness drive your actions rather than just taking ownership of things, and you start to live life. Small things bother you less. Big things don't explode so fast. You stop to ask questions rather than react. It becomes a domino effect if you let it.
Don't get me wrong, we are humans and humans get angry sometimes. Hell, I just got PISSED the other night simply because my dinner plans to make homemade pizza were ruined when I discovered that the sauce and pepperoni I bought were both moldy! (Yuck!) But I started to realize that I was getting snippy with my wife and even the dogs, and that my anger levels were off the chart given the circumstances. Sure, good reason to be disappointed or even mildly upset, but not PISSED. So I took a moment to go upstairs, breathe deep, and gain some perspective. I realized I wasn't going to die if I had to eat something other than what I planned for (I was mostly upset that plans changed last minute and stuff like that can throw me off easily) and that it certainly wasn't my wife's fault or problem (or the dogs). But I gave myself permission to be annoyed to a certain point, but decided to "take it down a notch" however I had to, and within the hour I was able to chill out and no fights took place. I also took a moment to share my frustrations with my wife and just let her know that I knew "it was just me", so that she didn't have to feel like I was unaware of my own frustrations.
You don't have to be perfect. You just have to be someone you are proud of at the end of the day. If you aren't proud of who you are and how you reacted to things that day, then make a decision to change that in the future, and then DO IT.