Feeling Blue

Do you ever just feel glum for long periods of time like there's nothing you can do about it?. Im 17 and currently studying a levels for the last year. My life is fine, ok so I don't have many friends, I never go out on the weekends, I'm avarage at college and I haven't got the effort to try hard and get the grades I need. My mum always tells me "12 more weeks and it's all over! No more revising! You can go have fun!" But honestly I just think to my self, will I? I don't have many friends, my jobs pays a very low amount so I can't really afford to travel or anything and I have to go to uni after my gap year. All I want to do is join the Army, but I want to join as an officer so I need a degree. Everyone tells me your teenage years are the best, but I'm just glum. My life isn't tragic and horrid, it's just fine but fine doesn't make you look forward to getting out of bed in the mornings. Opinions?