possible trigger-depression

I'm 20 years old, been with my fiance for 5 years. I got pregnant at 16 had him at 17. then at 19 I was pregnant again. Now I'm 20 and pregnant I'm 12 weeks. I'll have a 3 year old 1 year old and newborn in December. I just have become depressed ever since I had my first. I haven't talked to anyone about it. I guess it was post partum that just developed to normal depression. I never do my makeup or hair anymore I never go outside. I used to love doing all those things. now I just sit in the house unless I'm taking my kids to the park or something like that. it feels like a huge chore just to take a shower or clean the house. I feel like a failure. I dont know what to do my mom told me to get over it and my fiance doesn't understand it. where do I start? how can I get rid of these feelings?