Are you on any antidepressant? If not you'll have to REMIND your doctor of the diet pill that you're on because the antidepressant may NOT react well with it.

But it certainly sound like either your antidepressant is NOT workiing well, or it needs to be increased in dosage or changed altogether. You need to talk with your doctor about this, and do it soon! Having these dark, suicidal thoughts is dangerous - so please don't delay, okay?
As for taking the diet pills,, yes you are playing with fire. People react differently to diet pills, and these may be contributing to your dark thoughts. These are obviously an "upper" and that's NOT what you need.
Please talk with your doctor about the whole ball of wax, okay? I'm sure you and he can come up with a solution to this problem. Right now things are just out of whack. God bless and please take care. Hugs, Lee