I'm sorry to hear your mom misses your events. I don't know you or your mom or your parents' separation situation. But I would just say that we only get one set of parents. They won't always do things the way we think they should. Often, their actions will hurt us a great deal. Bear in mind that we do plenty of things that hurt our parents a great deal as well.
The thing is, you may have to cut her some slack if things aren't going that great in her life. Even under the best of circumstances, being a mom is the hardest job a person can ever take on. And under less than perfect circumstances, it can be down right impossible.
I know in my job if my boss required my presence, there would be no saying no. I'm sure your mom would much rather be at your concert than at this meeting. But she has to keep her job in order to make a living.
Let her know you would love to have her there. Then accept that she loves you and is doing her best right now.