You might be far enough along to read "How Can I Forgive You" by Janis Spring.
She has some really good thoughts on earned forgiveness, and how allowing a good relationship to be built between you and the offender is not accepting, condoning, or "letting them get away" with their bad actions.
This book is not for people just starting the journey on R, but I really understand where you are coming from.
Unfortunately, the reality of the matter is that cheating is just ridiculously unfair. The repercussions of it are felt in more enormity by the BS, even if there are some social implications for the WS. There is no civil, criminal, or other penalty for cheating anymore in most jurisdictions. If you choose R, to some degree, you have to live with the unfairness. This is often called "eating the shit sandwich" around here, and yeah, it sucks.
As for how to deal with lack of motivation related to personal goals, I suggest regiment. It's much easier to "just do" whatever it is you are trying to do if instead of being "motivated/inspired" to do it. I have to set time aside to actually do it. Consistency is more important than intensity. If you want to write a book, set aside an hour every day (or whatever fits in your schedule) to do that. Then at that time, do it.
To be totally honest, you will have more bad days than good, but by just keeping at it, you are much more likely to finish than if you wait until you feel internally motivated to do it again. This is basically what I do with exercise, language learning, and other projects (no current projects underway, TBH). I say "From now until [finish date], I will do [activity] during [time slot on selected days]". More days than not, I'm running slower than I would like, struggling more than I would like, etc. But it's far better than not doing it at all. "I'll just work on it, in my spare time, when I feel like it" is a slow road to nowhere, even if each individual session feels like a triumph. You just won't have done it enough. I'm not suggesting you turn your free time into "work" but if you want to do something substantive with your time you have to do that thing consistently whether or not you want to do that thing in the moment.