Everything you wrote about your concerns about the BD party is valid. You are NOT RESPONSIBLE for your H deciding to have an affair and leave. Not being invited to family birthday parties at your ex's home is one very small consequence of doing what he did. I really think this is another example of you shouldering the extra emotional burden-- the difficulty of the situation itself and perhaps it causing backsliding in your own healing-- just so that your H doesn't feel badly and suffer the consequences of his own choices.
Also... don't do the "if the roles were reversed" thing here. I don't think that makes sense when you're dealing with someone like your H who has made the choices he's made. He has missed birthdays and even the BIRTH of a child before! It doesn't matter to him like it would matter to you, beyond the "he made his bed" part of it, which might feel a little yucky/petty to you right now. (though there is a part of me that feels the more you enable his cake-eating, the longer before he is motivated to do any work on himself. And for you, the longer that shark-eyed version of him is around, the worse off you'll be in coparenting with him and sharing a business together.)
Don't do it. You deserve a fun and stress-free birthday party for your oldest child. There is enough stress in the world right now that if you can carve out a little time just to have fun, you should. Remember how terribly he behaved at the last one?
If it is too hard to think about doing it for yourself, do it for your kids. You know you're a better parent when you are feeling detached and compassionate and whole and healthy. Say no to your H as a gift to them.
(((Sage)))
Me (46) H (42) M:14 T:18, D9 & D11 4/19 - 12/19: series of escalating BDs 9/20 - present: R and piecing
