Depressed and alone

From a young age I have been an incredibly happy person. I went through a lot as a child, things that I wont mention, but I got through the harshest of situations with a smile on my face. As I got older, I seemingly became weaker. Just a small problem could make me come crashing down. Now, I'm 20. I have the best fiance ever, currently living at home and my family life is quite good in comparison to the past, I have a job and I am looking for a full time job... Everything seems pretty damn perfect. Yet I wake up, not wanting to get out of bed. Not wanting to join in family conversations. Not wanting to hug my man. I just want to lay down. I exercise a hell of a lot, eat as healthy as possible, constantly crack jokes in attempt to make myself and others smile... but I still feel totally empty inside. When I do feel emotion, it's pure sadness or hatred toward myself. I cry at Dexter season 2, for god's sake, because the woman keeps getting her perfect chest out. Is that how pathetically insecure I've become? It gets worse. I was up all night on my mans Facebook looking at pictures of him happy with his exes, just for it to obviously hurt me. I'm still thinking about the pictures now. He's done nothing wrong and I'm not mad at him so why am I thinking about them? What the hell is that?! The problem is I know I'm the problem. I can't really describe how I feel. I guess I feel disgusting, unworthy an stupid, yet I have no reason to feel that way. My man tries to talk to me. Sometimes he questions too much or just doesn't understand and we end up arguing so I try not to talk to him about it in order to keep us okay. I don't trust my mum with my problems because I know she'll turn it into a gossip fest and my brother won't care. I don't have any friends, really. Just people that use me for a conversation. The only friend I do have just tells me to cheer up and calls me stupid if I admit anything like this to him. I don't know what to do and I feel myself worsen every day. I need help but I don't know...