I feel like I don't have anyone.

My fiancé and I welcomed our third boy (his first) December 8th. I felt on top of the world and in love. My other two boys are from my first marriage and I have split custody. Slowly since my third boy was born, I have been having crying spells... but not about the baby, but rather feeling useless and sad when my fiancé will get grumpy with me. I will begin to feel like a child who has been scolded... and that's when the tears start. 
Things have especially got worse for me... my boys came home from their dad's house and got to meet their brother. They are so excited... but my fiancé is treating my boys, much differently now and making it difficult for me. Last night, my 5 year old woke up crying in the middle of the night... he was throwing a tantrum, but obviously he was still in a sleep like state. I asked my fiancé to help me out by checking on him and seeing what he needed as I was breastfeeding our newborn. That's when things got worse. He started yelling at my 5 year old and started punching the couch. I've never seen him so angry out of the 4 years we've been together. But of course this makes me stress out and I end up having to jump in and calm down my 5 year old. Took some time, but found out the reason he was crying so much and I was able to get him to calm down and back to sleep. 
Last night was the night I realized I only had myself to depend on. Now after realizing this, I am utterly depressed. I don't have family near and if I did, they wouldn't care about me... my friend is 3 hrs away and we don't really talk about feelings much.  
I keep thinking of how much better life would be without me. I keep thinking that my kids should go back to their dads house and I could drop off the baby with my fiancée mom and I could just disappear. My fiancé wouldn't have to deal with my bratty kids and wouldn't have to deal with me. 
Thanks for letting me just let someone know, even if you don't know me or care.