Depression Support Group
Depression is a real and debilitating condition that is often misunderstood by family and friends. Its meaning can range from a prolonged period of sadness to an actual mental illness with specific symptoms. Find and share experiences with others who are going through the same struggles.
Beyond that, .. I doubt you and your step Mom can have a positive relationship. Maybe, but I doubt it.
Seems to me the best thing is GF's 'boyfriend' faces a paternity suit for child support and she takes on the single parent role, .. or PERHAPS adopt out. Dunno what her qualifications are for being a parent. Perhaps you and she could 'marry up', and raise the child good and proper ... I would not know. She needs to take a class in birth control.
You need to place blame where blame belongs .. as if blame does much good. Living with step Mom wasn't a very good idea in the first place it seems. You need to secure a decent income and provide for yourself ... and MAYBE a significant other and a newborn.
Convert this train wreck into something wholesome ... at least for yourself. Just my opinions.