why do I feel sad

I am having a hard time in life right now. I know I have been depressed for many years now. Some days even months are better then others but that old familiar feeling comes creeping in then it knocks me down like a wave. I don't feel suicidal, but I cant seem to feel happy. I make it seem like I am happy so my family doesn't think something is wrong because if they did who knows how they would blow it out of proportion. I don't enjoy things like I use to I feel burnt out almost. I recently had a daughter 3 months ago and became a parental figure to my boyfriends daughter 2 years ago. I have lost a lot of my friends along the way. Not to mention I decided to be baptized and many people who turned on me because of this, a lot said I wasn't a christian because i didn't seem like one to them. Basically, I have no one to talk to. I'm not sure if this is a transitional period in my life or if I am actually going through something.