OW doesn't matter because really this isn't about her.
I spent far too long on OW and put too much energy into obsessing about her. Time and energy that in hindsight would have been much better invested in building on our R.
If there had been no problems in the M your H would not have looked at OW - it just wouldn't have happened. The important thing is to work out WHAT went wrong, and if you can and/or want to, sort out those things.
OW is a symptom - not the cause. My H 'thought' he was in love with OW but that was because of the adrenalin rush etc. He wasn't. He will admit that readily now but in the throws of it all they just don't see it. OW is an escape from the problems - a 'get out of jail' card - but she is not THE problem.
Do you see what I am saying?
You don't have to 'forgive' the OW or anything. Ignore her as much as you can. Don't spend energy on fixating on her but instead think about what you can do to 'attract' your H back if that's what you want. At the end of the day though you are working on YOU. Any changes must be for YOUR benefit - otherwise they are not 'true' and you won't be comfotable with yourself or able to sustain them.
Honestly, it isn't about the OW. I know it is hard to see that when you are in the midst of all this, but truly it is not about her. It is about your M and the dynamics within it. It's about what brought it to the place where your H was willing to include this 3rd person in the R - not about the 3rd person themselves. If it hadn't been that OW it would have been another. Something made your H feel it was ok for him to turn to another person - that 'something' is what needs addressing. It took time to get to that point and so it will most likely take a similar amount of time to get things back on track.
Saffie me 46 H 46 M in 1986 D20,D18,S16,D13 H's A 01/05 to 07/06 H recommitted to M 07/06 renewed vows 09/06 Going from strength to strength