I think your decision to go to the birthday party is right. not because it is what your ex wants but because it would make your D happy.
And I really don't consider it "cheating" if your ex started dating after you were separated but had not filed yet for divorce. She had declared the marriage over. Many of us (myself included) had to figure out that our spouses started cheating long before they left then they left us for that person. That's a little different. telling us all is good but living a different life.
You are not over your ex. It takes a long, long time. And it hurts like hell. I have been there and I totally get that. Just keep working on yourself. Plan your life without ex in it. As much as you wanted her - she wants something else. Hurt Hurt Hurt.
Consider yourself one of the lucky ones who can still converse with ex. Can still be invited to a family event with her and your children. Many of us have to deal with our ex living with the person they ran off with and in my situation - we NEVER would get together with them and share an occasion with our children. Our children NEVER go there either.
So please accept what it is at this time. Work on not thinking about what is going on in her world, but what is going on in yours. I recovered much better when I remembered that God only gave me one life and I wanted to fill it with the things I wanted to do the most. And I did and I continue to make those plans. When I found myself thinking about ex - I used the Stop Sign technique and redirected my thoughts. I stopped fighting with him. If I had to deal with him - I smiled and nodded and stuck to the situation at hand (usually our very ill son).
C2H: things are going to get better. you will not regret going to D's party. I rarely regret the things I do - I often regret the things I didn't do.
