Saddened to learn that you're in this situation.
Nothing really to add here except that the people saying that the focus should be on the baby are IMHO right.
Especially until the PT, do what you can to try to keep your existing relationship going. Afterwards, if she's still willing to work with you, keep working on it. Sometimes the huge challenges turn into opportunities; if you stay together it could make your relationship stronger.
Stay on good terms (as good as you reasonably can given the circumstances) with the pregnant one. If the baby is yours, own up to your responsibilities to that baby, (sounds like you're already doing that / planning on continuing to do it) but don't necessarily commit to marrying the mother. Commit to staying in that baby's life - IIRC, you grew up in a single parent household and know how tough that can be, but don't commit to staying in the mother's life. If the baby is yours, maybe with time you and her will want to be together, but ending up in a marriage that you don't want because it's the 'right thing' will eventually most likely have you hating the mother, the mother hating you, or both. Not the best environment for a child.
Good luck with it. If it isn't yours, I hope the mother finds the real father and he becomes a part of the kid's life. If it is yours, well, it sounds like you are willing to accept responsibility - I hope that continues.