Yes, I understand where you are. And, yes it can get better - but that only happens by working on yourself.
You do not have control over the outcome of this relationship. But, operating in fear is not going to fix the problem either.
I was in misery probably most of the first year. It wouldn't have taken that long but I kept spinning my wheels and defeating myself.
There were many tools I used to finally start to crawl out of the hole I was in, but they aren't what you think they would be.
1. Deal with the lack of "happy chemicals". You are used to getting high from the cheating whether you realize that or not. This is a source of depression and you can't get better in that state. For me, I took vitamins (being low in certain ones can effect brain chemistry) and I started running even though I had never done so or liked it before. I found that if I did that 3 or 4 times a week it kept my brain stable. You may actually need depression medication from your doctor depending on how much you can do naturally. But, you are fighting a lot with your brain chemistry and you won't win that fight without the right tools. Do not hurt yourself, and do not hurl your shame and pain at your BS, you need to get stable so you can be her rock if she lets you.
2. Do not live in fear. To me, this means get a plan together for being able to be on your own. It's hard because these things can really activate past trauma, especially if you have abandonment issues. (And the same happens for the BS but it's much worse). So, this means IC to work on your healing but also really sit down and have a plan for the practical things in a divorce. Do not give up at this stage, but having a plan will help with Fear. Fear is actually your enemy right now. It will keep your propensity to manipulate rather than to be brave, work through your issues, try and make amends and take accountability. You can't get to those places without getting brave. Your wife just had her world blown up, get yourself stable.
3. Read, watch podcasts, take an active effort every day on learning, improving, being there for her. Figure out your whys and hows and work like hell on them.
I found for me, the more I focused on doing the right things day to day the more I felt better about myself. The more I felt better about myself the more stable I got. The more stable I got the better equipped I was to help my husband and our marriage. Do not wallow in this place, the more action and work you put in the less time you will have to spend in this mental space you are in.
It can and does get better. Embrace the learning and healing, and let go of the outcomes. Much easier said than done, but it's the only way through that I know.