Do I have manic depression, please help?

Every now and again I have a mental breakdown, and feel like dying. Sometimes It lasts for a few days, or maybe a week. Today it lasted for a few hours. I was fine when I woke up, but I was stressed about my boyfriend's birthday because I hadn't gotten him a present yet. I called my mum at her work to ask her if she was taking me to the shopping centre to get him something, and she said that she wasn't because I hadn't gone to school. I tried to explain that I didn't go to school because I wasn't feeling well. She hung up the phone though. I felt unbelievably angry, and started throwing things off the bench. Then I went to pick everything up and just started hysterically crying. I then started screaming, and begging 'god' to let me die. I then went into the kitchen, got a knife, and attempted to cut myself. I only put a small cut on my arm, and then went and put the knife away. Then I went into my room, cried some more, then fall asleep. When I woke up my mum was home and I walked into the kitchen, said nothing, and sat on the floor with a block of cooking chocolate. I started crying and told her I felt like dying, just as I had several months ago. She said it was because I'd been taking my medication at different times each day. We then agreed to go to the supermarket and get my boyfriend a present. After that I felt fine, but I'm so scared that I'll lose control and start acting crazy again. When I feel really depressed, I can't stand it. There are times when I'm really happy though. I'm so confused, because I don't know if it's just depression, manic depression, or neither.