I think the crux of it all is when you are cheated on whatever your circumstances are, you go through a huge roller coaster of emotions. You blame yourself, you blame your partner, you blame the AP, you blame your situation, you analyze everything. That is entirely normal and it is OK to do, whats more this is a great place to do it.
People tell you to not to blame something or other, just move on but to process your breakup yo need to go through this horrible raft of emotions in order to come out the other side.
Your brain is working out what is going on with your situation, how you will cope in the future and so on and so forth. You struggle but you need to, it's a new situation you need to get used to, change can be bloody difficult especially when it is forced on you. People often really resent change in any circumstance but when you didn't choose this life, it feels impossible and deeply unfair. We know life isn't fair but it doesn't stop you feeling hard done by. Two people who aren't you made a choice that makes your life and those of your DC if they are involved, more difficult. Those two people took away your personal agency, you have lost some control over your life. That is hard to forgive and forget in a click of your fingers. You have to navigate all those feelings of anger, fear, loss of control, grief, and shame, it is overwhelming.
It is often better to feel the full pain of the emotions as they come and not surpress them unless you have to. Stiff upper lip in the usual circumstances of course, other people, wider family etc are often affected by a marriage breakup too. It's not just two people it affects, it can even affect your workplace depending on the circumstances surrounding it.
Please don't tell the OP to not feel agrieved that the man she loved has found a willing AP. It's perfectly OK and natural to find them both reprehensible. The AP doesn't bear the full responsibility but nor has the OP said she does, however the AP does bear some responsibility, she made a choice to fuck a partnered man and she knew about his wife and children. If APs didn't exist in such numbers partnered cheaters would have to face their issues on a personal basis and or within their relationships. Instead he gets to bury his head in the sand because an AP has made a choice to enable him to cheat with her.
The technical details of who is right or wrong doesn't matter, what matters is yet another family is going through a terrible time thanks to two selfish people who both had choices and full agency. The OP needs to process that and she has asked for help and support to do that.
I hope OP that you have found support here.