UsTwo..damn, I read something of his and agree.
Ghoastgirl, same thing. You are young and will change a lot. I got engaged at 19 and it was to be a 3 year engagement. Yea, that didn't work...at all.
Third, you're too young and too fresh in a relationship to deal with depression, medication, and low sex drive.
You want to leave, you just don't want to do it. Realize that it's just going to get worse as time goes on. You aren't even married or close to it and fighting...it's not healthy in any aspect. End it now, it won't get easier as time goes on...
There are many red flags in your OP...I'd say look at them, discuss them, and carry on with your life.