Dealing with Depression Naturally

I have been taking the carbomazapine for depression and anxiety for 9 years. The doctors arent sure if I am bi polar or not the therapist both have said no. I have suffered with major depression for many years over 35 now. I decided to try b12 500 mg it is working great with my medication 250 mg at bedroom and the b12 in the am. they wanted to change my med but I don't want to. Why is the b12 working so well? I feel so much better, I am allergic to many anti depressants.