How to get rid of my mood swings ?

I am taking zoloft for my anxiety problems and it seems to take the edge off. I have been in therapy for a year, and I am getting so much better. One thing that I really cant understand is these funks I go into. I get really sad and just feel worthless. I dont know if its bipolar or what, but I was just wondering if there is a way to stabilize these mood swings I get.