I am in kind of a slump/mild depression. Any advice?

Im a junior in high school and my grades have taken a major nose dive(I have had a 2.5 just about every year). It is 2 days till summer so there is no way to get them up. I have truly started to fear that my ability to get into college is starting to become slim and i dont want my mom to be disappointed me.(I would be the first kid to go to college). I have promised myself that i will get great grades senior year but i fear that it will not be enough to impress a slightly above average college. It seems lately that i have no motivation to do anything. Any advice or at least some positive thoughts to make me happy?