Yes - "let it go" almost sounds flip (I know you know I didn't mean it that way).
If ONLY it were that easy. Hmm, think I'll let that go - and off you go on to bigger and better things/days yay!
Really what it is, is a process. You aren't on that path yet, you are right where you have always been, hoping she will straighten out, become the person you knew and all will be well.
That might happen, it might not - back to the more will be revealed.
In the meantime though, regardless of how that works, there was/is something inherently wrong with your interactions with her and probably has been for some time. I'm not saying that is your "fault" or hers in fact, it just seems like that is key here. Perhaps trying to unravel that will give you some peace of mind.
If you are going to be around her, you will need to deal with that anger because you are not helping her if that is what you are bringing to the table. Now, you don't have to help her per se, that's not your job and I certainly don't mean getting too involved but showing up to lunch angry is not helping any of you. So if that is your M.O. right now, keeping your distance would be doing everyone a favour.
Yes it's been an absolute disaster but she is trying and she has discontinued the affair (according to her) - if so, she is moving forward - you can too.
(good point Anvil - better wording would have been - blew up the family "as you knew it").