I Think Too Much

I was doing fine. Progressing in studies, making friends, involving in family activities. I was finally starting to believe that I am winning depression. I can finally be happy. It had been months since my last depressed moment.
But now I don't know why. It might be because I think too much. I'm starting to experience the same hollowness like before. I can recognise it. How I used to feel before.
I am getting sad frequently for no reason. I'm feeling empty for no reason. I'm lonely again. And this aches my heart.