I have bipolar. i go to therapy once a week and am medicated. still i break down in violent tears randomly. why?

Bi polar: Your medication are most probably not optimized as yet.

Bipolar: There are many different medications and combinations to try. If you are taking therapeutic doses( and have blood levels in range) , you should discuss alternative medications with your doctor. Some conditions are treatment refractory, meaning not responsive to the typical med regimen. Keep trying , you will find the right one.