Well I think the resentment is different for all of us, takes its own time to process. And ultimately our choice of when to forgive. I think holding on to that gives us some false strength to move on in our minds.
Its not really about moving on till we learn to forgive, ourselves as well as our spouses. But, we must learn to forgive ourselves first. The rest will fall into place. If you continue to hold on to that resentment, you'll soon find out you'll also be carrying it into future relationships. At least your admitting to yourself that your still hurt by the actions, that 90% of the battle. Its something you need to work on, in your own time.
As far at the B-day pickup. Be honest, you don't need to placate her anymore. If its your day, and you had plans, then just say you already made plans cause u knew it was your day. If you really want to attempt to honor her request, invite her to the birthday party. If she declines, its on her, not you. Don't get, or let her, caught up in a battle over custody. Its your daughters day, she has the choice to spend it with her or not at that point, your not depriving her of anything she cant choose to do.
