A few years ago I got into a relationship with an older (by 10 years) engaged man. I was an emotional mess and he also was in a bad state, although I didn't really recognize any of that at the time. When he finally had the guts to tell me in no uncertain terms that he did not love me or plan a future with me, I lost it and tipped off his fiancee. I didn't apologize for my role in the whole thing and now I feel really bad about that - about having pursued him when I knew he was engaged.
I know through mutual friends that they reconciled and have since married. Would it be good/OK to write now and apologize, or is that just stirring up old pain for no reason?
Answer: Oh, my. That would be (b) - stirring up old pain for no reason.
Apologies aren't a good thing when the apology does nothing to improve the quality of life of the person you wronged.
If you and the fiancee were essentially strangers, then what does she gain from having you back in her life, even if only on paper? You didn't know her personally, and so you meant her no harm personally, which she'd know if she paid attention. Your apology wouldn't tell her anything more, except that you felt bad. That doesn't leave you entirely without recourse. Doing something rotten that you know was rotten and that you can't undo or repair will feel, understandably, like a personal low point - but as anyone who has navigated out of the depths can tell you, it's part devastation, part opportunity.
Your impulse to apologize is an impulse to be good. So, find some other, private ways to be good, to balance out the losses you caused and then some. It doesn't erase your mistake, of course; all it does is change the proportions of what you leave in your wake. A preponderance of good reduces mistakes to exceptions.