If you can pull this off by speaking quietly and calmly, my suggestion would be to respond to her with something like the following the next time she brings up the baby or makes any other rude comment:

I appreciate your concern "girlfriend's name". I made what I felt was the best decision at the time and will continue trying to make best decisions for myself and my son.

Keep it short and sweetly stated. After you make your statement, walk away. You are taking good steps to make your future better. You are looking for a job, planning to start school in a few months - and you are contributing to the household by helping around the house. Take pride in those steps!

Change your statement words as needed for the attacks. Keep them short and keep repeating them every time she tries to put you down. This is the "broken record" approach. One of those tricks I learned while raising my sons which resulted in them quitting certain rants and behaviors. Since she sounds like a child, it just might work!