Well, you're the rare person who has me beat for how long it took you to come clean. I had D-Day 2 at the 29 year mark, so I'm familiar with the problem of having forgotten important facts that my BH needs to know. I'm also familiar with how you can lie to yourself and really start believing those lies over time. Your stories of how she's got it all wrong, there's no "there" there with SIL, you couldn't penetrate the homeless girl (as if that makes it better somehow) -- these are classic minimizations. We minimize to make ourselves feel less guilty. In the rare case where the BS actually has a detail wrong, we seize on that with outrage and resentment, using that to justify other lies ("it's not like she even believes me now anyway").
So here's what you do, if you are serious about helping her heal, rather than just trying to make it all blow over (which is impossible, and the sooner you see that, the better).
1. Timeline. A written account of everything you remember in as much detail as you can remember. Don't bother lying, because of the next step.
2. Polygraph. Yes, you can find one, even if it's a long drive. If it's too expensive, sell something you care about to pay for it. They can't ask you very many questions, but one should be if you deliberately lied or omitted anything from the written timeline. Thinking about where I would be in trouble with this question really helped me face the lies I was telling myself.
Note that if you really have forgotten things, genuinely, the polygraph won't register that as a lie. I understand that there are things your BS needs to know that are truly gone. I have finally reached a point of absolute honesty with my BH. We went through every piece of source documentation from the A, and there were facts we both forgot that surprised us, things we were stunned we could have lost or suppressed from memory. Some of that is, unfortunately, inevitable, but the poly will help you sift the real forgetfulness from that which is convenient or strategic.
3. IC. There are reasons why you cheated. They are bad reasons, because there are no good ones, but they are there, and you need to confront them. It's not just about making an action plan, and it's definitely not about figuring out what to post here so that your BW will believe you "get it." You need to learn about yourself, why you thought you were entitled to have sexual contact with vulnerable women in bad situations (inappropriate even if you weren't married -- a drunk abuse victim and a homeless girl). IC is about becoming better than that for yourself.
Your marriage may not be salvageable. Some BS can't get past the WS's decision to steal their life choices. But how can you even ask for forgiveness if you're still actively stealing them? Your instincts are telling you to lie and minimize to make it easier, so there's less to get past. Your instincts suck. Mine did, too. Lying just adds to the mountain of shit you already have to shovel. Use every tool you have to force your head around to face that. It's always the hardest advice for WS to take, but the single best piece of advice that I can give.