My d-day was 12 years ago, and I've been on SI almost that long, and I urge you to keep SI to yourself for now.
I suggest, in addition to NOT "Just Friends", reading https://survivinginfidelity.com/topics/324250/things-that-every-ws-needs-to-know/ and seeing if it makes sense to you.
It made a lot of sense to me, and I printed it out when I found SI. I cut off the footer of the printout, because it referred to SI. I gave it to my W for her to read. I think it helped.
*****
If I leave her it will turn my world upside down in so many ways. I want to make sure that I am ready to leave before filing papers.
I agree on not making decisions until you're ready. That may take longer than you want it to take, but my very strong reco is to let the decisions grow - do not force yourself to choose one way or another.
Also, my bet is that your world has already turned upside down. Don't discount that - you've been betrayed and that's traumatic. You're in a new world. You don't know much about that new world yet, but you can and will learn. Have faith in yourself.
IMO, every BS needs to teach themself that both D & R can be good, honorable outcomes leading to a good life. You may want to go one way right now, but both outcomes can be great - and sometimes you need to take time before deciding between them, even though some of us my press you to act now - 'ready, fire' instead of 'ready, aim, fire'.
If I stay with her, I want to be confident that she has broke off all contact with this man, and that I can go through my days without feeling the need to track her every move.
Gently, you need a lot more than that to R.
The guy isn't the problem; your W is.
To stay together, my reco is to require honesty - no more lies and answers to every question - as the 1st, most basic item.
IMO, to R, your W has to change herself from cheater to good partner. She needs to commit to doing that; otherwise, R won't work.
*****
IOW, the way to regain trust is to let your W earn it back, if she's willing to do the work. If she doesn't do the work, she won't earn trust back.
*****
My reco, because your W is denying a basic fact (slow and deep), is to consult a good D lawyer or 2 to find out your rights and duties and likely settlement, because D is best if your W continues lying, IMO.
Get tested for STI - they do that (for many STIs) if you give blood.
For sure inform OBS.
I don't recommend exposing the A unless you split and someone asks you why. Exposure may cut down your WS's options. You want her to choose R because she wants you. You do not want her to choose R because she feels cornered. R works best if both partner choose it freely.
*****
More recommendations:
I don't think you need to do much more detective work at this point. You know she betrayed you emotionally and probably betrayed you physically.
Your next step is to figure out what you want. Figure out if you want R. Figure out your requirements for R, if R is on the table for you.
Once you're figuring those things out, the difference between a PA and an EA may be significant. If it is, that's the time to consider a poly - but only if you'll R with an EA and D with a PA.
*****
I know this is a lot to consider. All you can do is go step by step - and you choose what steps to take and when you'll take take them.
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ETA: Your W is probably the same person now that she was before her weight loss. Her weight loss probably just got her in touch with a side that she kept hidden perhaps from you and possibly from herself. My W had a dark side that she talked about but that I could not comprehend. She met the wrong person, and her dark side became much stronger than it had been, and that enabled her A. It was always there; she just never knew how to tame it.
[This message edited by SI Staff at 7:07 PM, Wednesday, January 11th]