I have 3 personalities.... and thoughts that wont go away. Help!?

Okay, so i have this weird thing, were sometimes i act like i want to be a person that's cheery, funny, nice, and bubbly like a little child. I also get really clumsy...(BUT not on purpose. Do you think id actually LIKE to hurt myself in a stupid way, like tripping over my own feet? >.<). Another times i feel like depressed, or acting cold and mean, ignoring others, and i have thoughts like hating my best friend. There are times when i turn innocent, shy, and really scared of how others think of me, up to the point where i would freak out and start crying. Usually im shy and scared when im in a new place, or a new environment, and i dont know anyone. This makes it really hard to make friends. I even start crying too. I dont act differently depending on who it is, i just switch personalities because the thoughts of each personality suddenly come to me, who im with doesnt affect it or change it. I dont want any mean answers.. because im serious. Usually im the first one, but there are times i turn mean, and then depressed. But im not "acting" it all out. When ever i act like one of personality's, its as if its really my personality. I also really hate it when i have mean thoughts of my friend, and i get mad at myself for it, but i cant help it. I truly do love her(as a friend) but those mean thoughts just wont go away sometimes, and i hate myself badly for it. I dont even deserve her as a friend for thinking like thinking like this. She's super nice, and im happy when im with her, but theres times when i get really annoyed at her.. or i just want to stay away... and when i do, i try my very best not to show it because at the same time, im telling myself to stop thinking like that. Whats wrong with me? Do i have an illness or something? I really hate myself because of this. Dont be mean please.