Hi there...sorry to hear that you two aren't together, but let me state that I had an A, so did my H, we are D now, he lives with hos OW. I didn't know he was cheating, until we were getting D...
You know that the OM is not the issue...never will be, but it's how you react to it - that matters.
You need to focus on you...changes you need to make that will improve the overall state of your M.
If you need to diet, change clothes, cut hair - do it. Keep a PMA every time...be polite to her, don't argue, don't accuse...so what, she's having an A or has in the past...people are dying in Iraq daily, never to come home again...you see how unimportant an A is. It's a red flag telling you both...the M is in trouble. Correct the problems or move on.
Work on all the issues that are against the R - change you in all this that led up to her having an A - do not worry another second about the A - she wasn't "pure" when you M her, so what is the complaint now?
I don't mean that in a negative way - but to impress upon you that the focus has to be on you and what you can do to make yourself the "love of her Life" as you once were.