What the hell is wrong with me?

My mood swings are driving me crazy. I am so unstable. I get into these moods where i LOVE life, I love everyone, I want to do everything, i want to EAT everything, and buy everything, and I can't stop smiling. And then, one little thing someone does, it can be seemingly sooo insignificant... like my boss treating me rudely, or my friend making a snide comment about me...and suddenly its as if a dark cloud has ped over my life. I start thinking about all the things wrong with me/my life, and they keep adding up until its this huge problem that i can't deal with.Then I just want to be alone. I often have suicidal thoughts ociated with these feelings, and have even gone so far as to write my suicide letter. I also use to cut myself but am over that now. I know i don't have bipolar because with bipolar your moods can't possibly fluctuate that quickly. Does anyone know what might be wrong? Maybe I'm just realllly moody or sensitive?