Why does my mother shoot my down every time I'm happy or excited?

Projection. Sounds like the psychological phenomenon of projection. She's likely treating you the way she treats herself. And she's also likely to be unaware of what she's doing and how it impacts your life. Learn that it isn't personal, that it's about her, not about you, and you can develop some immunity to the pattern.
Mom's unhappy? It seems your mother has issues with her own happiness and until she solves it she won't be likely to enjoy seeing you happy. If that seems too simple, remember that some of the simplest things in life are some of the most difficult and troublesome things to deal with. You may , if the time is right, ask your mother about it. Don't present it as criticism and wait until things are relatively calm.