Take it slow and see what develops. It's certainly not the best of situation to be in, but it all depends on the maturity level of the both of you. If you're thinking about marriage later on, ask yourself whether you're ready to be a dad and whether you're ready to make some lifestyle adjustments. Kids are 24/7/365.

What kind of situation does your GF come from? Sure, she made a mistake becoming a teen mom, but is she now showing responsibility to get her life together? Does she work? Does she go out partying too much, or have a bad rep.? You have to take this all into account. If she's not responsible don't think for a minute that you can change her. You are making the biggest sacrifice in this relationship; I guess you have to decide if it's worth it to you.