depressed?

30 weeks pregnant today. Within the last couple of weeks I have been ridiculously sad and lonely. Just feeling completely worthless and stupid. My fiancé is doing everything he can to help me. He's amazing and I couldn't ask for a better man to be there for me through this. But no matter what he does I just feel alone and depressed. I've battled depression for years (actually about 13 years. I'm 22.) but I had it under control until I got pregnant and I just can't control it anymore. I know depression after birth is common but depression while pregnant.. is that a thing? Can pregnancy cause depression?