How to stay happy

I'm a FTM, my amazing baby girl is almost for months old. I love her to death but I can't seem to stay happy. My husband is gone during the day so that obviously leaves me and the baby alone all day. The nights are still hard so I'm not getting a good amount of sleep. I'm so sad and depressed most of the time that I just don't care about almost anything anymore. I just want to be my happy positive self again.