I grew up with this scenario. My parents split when I was 7, and they would not go to the same family party or event. In fact, they wouldn't be in the same room together for anything short of a dire emergency, and even then, they took turns. It wasn't the end of the world for me. I just grew up knowing it was their problem, not mine, and the non-attending parent was missing out on the fun. I also knew it was not my fault, and I couldn't fix it. (Which didn't stop me from occasionally trying.)

The good thing is that when we were all older (and presumably wiser) my parents did start both attending, and some healing between the two of them began. No, they never got back together, but when my Mom was dying, my Dad drove down to the state she lived in and helped her draw up her will. He even took his turn at sitting with her during her last days. So there is hope yet, that your ex will come around, but it may take 10 or 20 years. In the meantime, He's the one missing out on the fun, not you. And your daughter will know that Mom is always there to support her when she needs you. She needs to be told that you can't control her Dad's decisions, and it isn't her fault that he's not there, and she'll learn to live with that. I did, so did a lot of my friends whose parents acted the same way.

Make sure to stress the fact that it is not her fault in any way, shape, or form. Children believe the world revolves around them, and if something bad has happened, they somehow caused it. You have to take that one head on and repeat it a lot, it's not her fault. And hey, it's not your fault either.