Depression. I want to be happy again.

I had always been a good student-the topper since childhood but when I entered class 11th and joined the IIT-JEE coaching I couldn't cope up I mean I was the last one,although I used to study a lot but somewhere I just didn't go through it. The pressure was killing and somewhere I developed an unknown fear . Also my parents aren't keeping well since I guess 2008 they aren't their constant fights and differences has affected,although I don't show that it affects me but somewhere it does and eats my mind. Basically I feel lonely a failure at times and all. When I came into college I met guy fell in love with him like mad and he cheated on me and since then I just couldn't cope up.It has been 8 months since the break up but it doesn't get better. I always feel lonely. My grades are going down and which worries me a lot. I don't want to ruin my career.Please help. I want to be happy again.