I don’t think it’s necessarily wrong to stay friends with your children’s exes; it depends on the situation. I have a friend whose son cheated on his wife so they divorced. She told her son he was in the wrong for cheating, and has remained friends with his ex BUT, aside from that one conversation with her child, she refuses to take sides — she told them she is Switzerland.
In your situation… it seems a cruel blow for your mother to not provide you with any sort of emotional support. She could have stayed neutral while still encouraging you, giving you help in other ways and, you know, being HONEST that a wedding gift was a wedding gift. That’s not taking sides, that is just being honest. Instead, she bent over backwards to choose him over you, and that’s just wrong. My heart just goes out to you.
I think you did the right thing to be done with her. It seems her own choices colored her view of life in a way that was very unhealthy for you and you have to protect yourself from toxic people, even when they are close family members.
I wish you the very best.