I have an extremely strong feeling that you suffer from borderline personality disorder. Can you please ask your doctor about this or research it online? I have a few good friends who have BPD, and your experience is identical to theirs.
People who have BPD seem to experience emotions more vividly than regular people, especially when those feelings are related to abandonment. While there has been a lot of great feedback on here about the way normal people handle these feelings, they may not be working for you because you may have more complex issues that need special type of treatment. But the good news is it can be treated; you just need to know what the right tools are.
Personally, what I noticed about my two friends who have BPD is that they spiral in what appears to be an uncontrollable manner after a breakup. This is because during the relationship, they tended to base their entire identity in their relationship with the other person. When they are left alone, they do not know who they are without their loved one. What I particularly see in your case that reminds me of them is the obsessive loops of imagining that they know what the other person is feeling. Without special tools, they struggle to escape that loop.
I understand that you might not want to take the time or energy to look into this diagnosis, and it would be a lot of learning and experimentation to figure out how to cope during these periods of loss in your life. Especially now when you're feeling totally drained, that might seem like too much. However, maybe it will give you relief to know that this isn't in your head. This is probably a mixture of biochemical differences in your body along with formative life experiences where you didn't feel supported and loved. You're not crazy, you're not lazy, and you're not stupid for being unable to deal with your emotions the way other people do. This is literally the way you are wired.
But as you have learned, it is really difficult to go through your life with untreated BPD. It can be one of the most difficult mental diagnoses out there IF you continue to leave it untreated. I do know again from experience, though, that if you're willing to do the work, you can overcome a huge chunk of the thinking that is causing this bottomless grief and you will never have to face it again once you learn the coping mechanisms.
One of my friends who has BPD refuses to recognize her diagnosis (suggested by her doctor not just me lol), and she is getting more and more beaten down every year. She continues to enter relationships with the same mind frame, and she's not doing the cognitive behavioral therapy work to change the way she approaches her view of herself and others. I worry about her prognosis in the long run.
However, my other friend Amy discovered her own diagnosis, and she recognized herself in it. She has done so much work over the past few years, and she is doing so well right now. She's uncovered some significant subconscious beliefs she has about herself and love, and she has identified negative loops of behavior. She now knows when she starts to enter those loops, and she has found effective ways to stop that obsessive thinking. She finally found a healthy, loving relationship after a few years of working on herself, and she seems happy and stable.
I know that you are hurt, tired, and probably feeling hopeless about the future. I don't know what to say to inspire you. You may not have friends or family that you feel you need to do this work for. I personally think your dogs sound like enough of a reason. They would miss you. However, YOU are enough of a reason. Don't you want to know the rest of your story? It can't always be more sadness and pain. Look around in the world. Nobody's story is continuous sadness and pain. Who knows what the next chapter holds? It could be you doing the work and finding the most rewarding and happiest time of your life. And while I hate to define happiness in terms of relationships, you could just be months away from meeting the true love of your life who will make you realize what a loser your ex was. You might also just be months away from watching your ex totally crash and burn!
The point I'm making is you can stop this pain but probably not through the regular means that most people will suggest to you. You will probably have to find the special mix of medications and cognitive behavioral therapy that will help you to overcome the debilitating emotional disability that you suffer from. You wouldn't keep walking around on two broken legs, but hopefully you also wouldn't kill yourself for having two broken legs. You would seek out the right type of treatment and understand that eventually those legs will heal. Your heart and your mind are like this, too.
Please just take a few moments to look at the key symptoms of BPD to see how many apply to you. I know that just through what you told us here, I have seen at least three or four of the major identifiers in your reasoning and behavior. Just take a few minutes to read through that, and let me know if you think it might apply to you.