I think I might be bipolar, I really need advice now!!!

I think I might be bipolar. I go through stages where I'm very depressed, I just don't want to live anymore. I cut off all contact with people, delete facebook and turn off my phone. I stay in my bed and just cry. I start to get agitated, like I need to escape my body.. by this point I'm getting to impulse to cut my hair and cut myself. So I do it and it makes me feel better for a little while but then I get even more depressed because of what I've done and I feel absolutely horrible about myself. I get nasty and judgemental and I just don't want to talk to any one.. I can't see the point in life at all. Then after a few days my mood just switches, I'm totally optimistic and happy. I feel so much love for everyone and everything, I get very creative and I feel so lucky and grateful to be alive and for everything in my life. I start making life plans and feeling like I can do anything I set my mind to. This last for a while and then I go back to depression. Oh and if this makes any difference, I'm an 18 year old girl and I also have social anxiety but it's undiagnosed and untreated. I need advice!!!