My son is 20, and feels like you do. He has been diagnosed with Bipolar, and is on meds. The meds do not take away the severe ups and downs, but make them less often. He is a rapid cycler, meaning he can be way up or way down several times a day. Do you still live at home? If so, what do your parents notice?
You may be interested in reading my son's story...I wrote an ebook called Bipolar Monster, which goes through all the symptoms he had and what he went through from age 5 through age 20. You may see yourself in his story. I'd suggest, since you are 17, to let your parents know how you feel.
Good luck, and may God bless you as you search for help for your feelings.