Got married to first gf, now not so sure...

8 months ago I had no idea what I'd be doing with my life. Then, I got into graduate school and got married to my first gf. Now I know what I'll be doing for the rest of my life, and who I'll be spending it with. The lack of uncertainty is unnerving. The world just seems predictable and colorless...

My gf (now wife) and I have never gotten along perfectly. Now I am surrounded by attractive classmates, and when my wife and I fight I find myself imaging what life might be like with one of them.

Part of me wants to leave and try to start a new life. However, another part of me is afraid to be alone. I'm far from good looking and prone to depressed moods. Being in a relationship with her is the only thing that has ever cheered me up to the point where I feel like being sociable. I'm afraid that if I leave her, I'll be alone for the rest of my life.

Am I crazy for having married my first gf? Is it wrong to stay in a relationship just because it's a source of self-esteem? Has anyone else ever been in a situation like this?