The fact that you think you're fat when you're 5'6 and 125 tells me you're probably trolling. I'm 5'7 and 135, and have a good 10lb to go before I consider myself fat.
But in case you aren't, I'll just put in my two cents. I'm not sure how old you are - I'm guessing you're a traditional college student so 19, 20 years old. Your post struck a chord with me, because when I was that age I had similar issues with myself. I felt stupid, I felt ugly, I couldn't talk to people because I felt too stupid and ugly and gross. Everybody around me seemed so much more put together, more attractive, more articulate - I completely wrote myself off. I told my grades to fuck off - long story short I graduated with a degree I didn't care about and a sub 3 GPA. people think my shitty GPA is from partying, but I actually just spent entire days in my room doing nothing, just watching youtube videos and shit. I could not for the life of me step outside my comfort zone, and as a result my comfort zone got smaller and smaller, because I let my poor self image tell me what I was cut out for and what I wasn't. I couldn't say hello to people, I was so withdrawn. I was a pretty gray time in my life.
Before I start rambling, here's my point. Don't give up on yourself. Don't write yourself off. Tell yourself you matter, even if you don't believe it at first. DO SOMETHING. Force yourself to do something. You feel unattractive? Do something that makes you feel attractive. If you're anyhingn like I was, the first thing you'll say is "Nothing makes me more attractive" "I am stuck this way, nothing will change it." You are so fucking wrong. Go jogging 20 minutes a day. If you are self conscious, do it after dark (if it's safe) - that's what I did for a while. Do anything, something, regularly, that you KNOW you SHOULD be doing, but just WON'T because you tink you can't, or you think it's pointless.
For the love of god, you are young. Don't let these years go by. I'm 27 now, in a much better place with myself. When I look back on those years, I just can't figure out why I did all that to myself. It just wasn't worth it. I'd give anything to go back to 19, tell the negative voices to fuck off, and have happier memories of youth. Be kind to yourself, you'll have a freer and clearer mind to know what you really want out of life.