Look.
I've been 20, single, with a child, living with my parents.
You act like you've got it all figured out...but are you smart enough to stop, take a look, and realize we are ALL telling you the same things? Do you realize how rare it is for everyone posting to agree on a topic? Do you really think that we are all wrong, and you are entirely correct? That might be a bit narcissistic, don't you think?
Not once did I leave my child for days/weeks/months at a time. I went to school and worked and got my own apartment after my parents helped me. You say you are sacrificing. What? You are sacrificing...well, nothing. Others in your life are sacrificing because of your choices. Your mom who I'm sure didn't expect to raise her grandchild. Your poor dumb fiance who is about to support you. Don't you have any respect for yourself? Don't you want to better your life and yourself?
You are young, but that stops being an excuse when you have another life to take care of, and you are NOT taking care of her. $800 a month? You know, at Target a couple of days ago I bought (for kids) 3 pairs of jeans, several t shirts, underwear, a couple pairs of clearanced shoes, pet food, and groceries...and spent over $300. That is in one week. Granted I have more kids, but do you see where I'm going with this? Kids don't get cheaper as they get older. Your $800 is practically nothing in the whole scheme of things.
You aren't getting it. And it doesn't sound like you are going to until you take a look around you and realize some facts, not just your own set of made up, partially true facts that you are viewing through your colored contacts. You have made some really poor choices, and you are about to make more...