Am I being wrongly diagnosed.?

I am 29 years old and am currently on sertraline anti-dep tablets. I have been on and off different tablets for months and still feel very down and suicidal. A couple of years ago I had a break-down and was diagnosed with severe depression. Things seemed to get better for a while then i started getting down again, sometimes I feel really happy and positive like I can do anything then just like a flick of a switch am down again and have feelings of fear. It feels like the happier I get the worse the come down. I just wana feel normal. Can anyone help