Heidi, 
I feel for you girl.  I’ve been there.  My son’s father walked out on me the day I told him I was pregnant, and I was so scared and alone.  If you absolutely dont think you can handle it, there is always adoption, but my best advice would be to go after your childrens father/s for child support.  Your babies deserve that money.  Also, there are programs out there to help you.  I made it through Nursing School as a single mother by relying on FInancial Aid and also signing up with United Way, which paid for my son’s day care while I was in school.  I know what its like to be a young mother with several children.  I just turned 23 and I just had my 4th child.  Just dont give up.
Good luck hon, and I wish you a healthy baby.
Christina