was having a good streak.. now not so much

I have been diagnosed with depression, generalized anxiety disorder, and PTSD from past sexual abuse.. I was having a really good mood for the last couple months and now all of a sudden I'm right back where I started it feels like.. I'm scared, down, and I don't even know why.. it's like it just goes in waves and there's no way to stop it from coming back around.. I hate being like this.. why can't I just be in a good place for a solid 6 months at least..