How come I don't feel so happy anymore?!

I feel kinda down I've done everything in my life I've ever wanted if there was something I wanted I was very determined to get it. I survived school I used to be fairly popular at school I had lot's of friends. Then I left school and worked for a while and got a mental disease which effected my life for a couple of years then I bounced back and done some courses and I'm studying to become a writer and I work part time. But I just don't feel complete. I have two best friends and I often babysit my nephew two times per week. But I live on my own and I just feel so empty sometimes even when I'm with my friends I always feel as though everything is a struggle or is too expensive remember I'm only working part time. I just don't feel as happy as I used to be do you think I could suffering from something? Because I feel like this emptiness alot and I don't know why?