I need help with me self esteem please?

I dont know why i feel this way and how it started. I could have the best day and wake up in the morning the next day and feel depressed and down for no apparent reason. I have nothing to be down about, i have a gorgeous girl talking to me, athletic built guy, going to college, nice things and clothes, a job. I have nothing to be down about but it is just a constant thing for me to keep feeling down and depressed. Also i somehow brainwashed myself to think i have no social skills what so ever. I made myself think that i cant hold a conversation at all with anybody so im constantly quiet and wont say a word because i feel that i have no social skills so why speak, one problem i do have is i think to much about the past and future to much and not the present moment more. i think about negative outcomes to much but i try not to its just a habit that glued to myself. I never in my life use to be like this, i was always goofy, funny, social and down to earth. people loved me and thought i was a really cool guy in high school. I'm 19 in my freshmen year of college. I dont know where along the road i started feeling this way but i wanna change back really bad to how i use to be. Im constantly surfing the internet on my cell about how to be positive, more social, stop feeling depressed,and the list just keeps going. I dont kow if constantly looking up articles like that is helping me or making me feel worse about myself. i d ont love myself like i use to and its depressing to think about. I wanna say what i want to say and do what i want to do and be who i want to be without having to worry and strssing about things that i shouldnt be worrying about in the first place. im getting really irritated and feel i have no hope of this getting over. i just want someone to help me figure this out or get some tips on how to get this out of my head. Its ruining my life with friends, family and everything in general. Please help me.