You don't get to quit now because you haven't seen the whole thing yet. Your whole life that is. When i was 20 i was very suicidal and told almost no one, i could barely muster the energy to get up and do basic daily things. I was depressed and embarrassed about it and afraid to tell anyone. I came really close to dropping out of college.
Then some things picked up for me. I got a girlfriend, joined a fraternity, got an ($8/hr - woo!) job in the field i was studying. I was happier, at the very least distracted from my depression, then things gradually got better. Then about a year later all that kind of blew up - lost the job, the girl turned out to be psychoclincgy, the fraternity was full of hyperexclusive toolbags, so i rolled with the punches. The girl was psycho but the sex was good and she was a nice girl, the frat was full of dbags galore but there were still some good aspects and a couple really cool kids that i'm still friends with.
Then by my 2nd senior year (23 years old) i had a new, better g/f, a better paying job in a nice restaurant, still in the fraternity but didn't feel beholden to the whims of the toolbags 'in power'. I sold bud, got laid, got a 3.4 GPA, and got lots out of life. I was genuinely happy.
Then...that all sort of blew up...graduated college, left my fun restaurant job to "be responsible and adult like" started working a full time job i hated....stayed in it for 3.5 years because even a crap sales job was tolerable and even enjoyable on Percocet...then i lost that job and was happy...then i was scared for my future...played poker full time to support myself and liked it...then 9 months later had to give it up to go back to the grind...and i was happy for the first 6 months...then i hated it...and on and on.
I'm 29 now, and i still get depressed here and there, but when i look back on my life, i had to go through all of the periods of depression that i did, i just couldn't pull myself out, but i LEARNED a little each time. Learned to cope and shorten the duration and severity. Many, MANY times in my life i felt like the load i was carrying was too heavy to bear and that suicide was my only way out. Somehow i trudged through it one day to the next, and eventually those days turned into 6 months or so, and finally something would give, and the doom and gloom would be lifted.
***Sorry to kind of tl;dr here, but the point is this: Your life WILL have always have negative aspects to it, and you WON'T always been in control. Believe me, EVERYONE and i mean EVVVERYONE in this world struggles with inadequacy feelings of some kind. Everybody gets down on themselves. Plenty of the people you see "always living it up" on Facebook feel the need to interrupt their 'awesome times' for photo ops to post on Facebook as if to say "see, i'm having fun, this is fun, right? I'm cool guys, right?" Not saying that's always the case, but believe me it is the case with a lot of people.
Try to let it all wash over you, don't kick yourself and feel depressed because you feel depressed. I've done that too - a little down in the dumps and depressed then you make it 10x worse by wondering "WTF is WRONG with me, why am i so depressed, why am i a LOSER???"
Don't compound the depression you're feeling by being further down on yourself for feeling a little glum. It's easier said than done to get out of that spiral, but just own it - "Yup, i am DEPRESSED as hell, but fuck it, it's not going to beat me. I'm not going to let this feeling be my undoing, not going to give up my life to this evil demon that lives inside me. My depression doesn't get to win, I DO. Maybe i only win 1 out of every 100 right now but goddammit i'm not going to just give up."
Good luck man, i'm totally rambling at this point but try to accept the things you can't change, and change some of the things you can.
You might benefit from getting on some medication, even if only for a short while. If the thought of that just makes you angrier, you should consider finding someone to talk to. A 'therapist' for lack of a better term. It's just a totally non-judgemental person who will listen to everything you have to say and usually gives very little feedback, but is listens intently so that they can repeat what you're telling them and give you an impartial perspective on your life that isn't clouded by all the muckity-muck you feel on a daily basis.
*Edit - Sorry this post is preachy sounding and sort of disjointed. This is what has worked for me and i figured i'd just share my experience. Good Luck Man, keep sticking it out and i promise you'll get to see some good things in life along with the bad and the ugly.