Constant mood swings, irritability, and outbursts of anger?

[Emma]

For the past 6 years I've had trouble with constant mood swings, irritability, and outbursts of anger. Sometimes I can feel so down I'll be suicidal, but it only lasts an hour or two. Then I feel perfectly fine as if there's nothing wrong. I don't think I'm bipolar because one of my friends has bipolar disorder and he describes manic episodes of feeling extreme energy and excitement. I never feel this way. When I am in a good mood I feel happy and satisfied, but never manic. Sometimes my mood swings are worse than others, but for the most part they have been a constant part of my life for the past six years. When I am in a good mood I don't think I need to find help and when I'm in a bad mood I always think I should see a psychologist, but I'm in too bad of a mood to do anything about it. My relationships with people tend to be rocky because one minute I'll be telling them how much I love them and the next I'll lash out at them and say something or do something I didn't mean. When this happens I feel out of control and I always regret it later. I also suffer from delusions and irrational thoughts. Does anyone know what could be wrong with me? In case it helps, I'm an 18 year old female with a history of self-injury and low self-esteem. Thanks for your help.