Hi
Anonymous22,

I'm very sorry that you are going through this tough and puzzling situation. You have to be experiencing a lot of emotions going on inside of you through all this. For some reason he is projecting his feelings, trying to unload them onto you. Did his ex wife have affairs by any chance?
Please remember that you don't need to engage his thought process or deny or defend at all. Here is a good link:
Don't JADE I know it's easy to feel that need to defend, but you don't need to.
I also believe it's critical that you support your children (and SD) through this and yes, do go to their games/dances. The insecurity your SO feels doesn't have anything to do in reality with the activities your kids are involved in.

Wools