Hi Carrie,
It sounds like you've been through the wringer, emotionally.

To me, this would suggest that you may have a mood disorder, and if your moods are changing that drastically for no reason at all, it's possible that your brain chemistry has something to do with it. There could also be a physical cause -- like a thyroid problem -- that's making you feel this way. Please do see your doctor to get any physical causes ruled out, and once you have, start addressing your depression itself.
If you're having suicidal thoughts, then something is seriously wrong. You are NOT weak for having those thoughts or for feeling the way you do. LOTS of people feel that way, including me, so I know what you're going through, and I know from experience that simply trying and "thinking happy thoughts" does absolutely nothing. I agree with stefano -- don't talk about your feelings to anyone you're uncomfortable sharing them with. If you can't talk to your family and friends, then talk to a professional, even if it's just to call your local suicide hotline to talk to someone there. On those occasions where you are feeling so suicidal you don't feel safe, please, please get to the hospital. Your parents will not be disappointed that you are taking steps to save your life -- but they WILL be crushed if they lose their child.
I think you've explained your feelings really well in your posts. Why don't you print them out and take them with you to the doctor? I liked stefano's suggestion of keeping a mood chart. Maybe by tracking your moods you'll be able to get a better handle on what might be triggering them, like weather, changes in the season, particular things that come up in your life, etc. As for what to do in the short term -- and I stress
short term because I really don't believe they're going to rid you of depression completely, only professional help will do that -- try to be good to yourself. If you feel your mood starting to slip, stop what you're doing if you can, take a few deep breaths, and try to clear your mind. Light exercise has been shown to lift mood, as has a balanced diet. And pampering yourself never hurt ... a bubble bath, listening to your favourite CD or watching a movie (nothing sad, though!!!!) always help me.
Good luck! I'm glad you posted, and I hope you keep us updated on how you're doing.