You don't need to answer to us but you should be honest with yourself about a couple of questions:
1. Did she actually call you her boyfriend? I know you guys have been good friends since early High School, but Ladder Theory (check it out) says that you cannot go from being "good friends" to "lovers" in a woman's mind - two separate ladders. How she defined your role in the relationship was awkward, right?
2. Did the entire relationship revolve around the Baby? Somehow I doubt you two hired a babysitter and went out, or left the kidlet at the Grandparents for the night. It was a threesome and between work, her needs and the baby - there was a lot of charity by you, right?
3. Did you guys actually have sex? First, she was all hormonal and pregnant, then had the baby and was healing down there, then she was nursing and . . . well in front of the baby? Something tells me that you maybe got to cuddle and snuggle in bed with your pajamas on, but you were not experiencing Rule 34 madness with this woman, right?
4. Did she really plan to stay? While I know we are all powerful and stuff, I bet it was her idea to move back home, and more likely she knew all along she was going back. The bad boy is just a bit player in this element of the story - the bad boy that she says she hates, but makes such an effort to be so entangled. Someday you will find it was all a scam.
Son, you've dodged a bullet. Time to go for a long walk with your Dad and revisit the conversation where he told you this would happen. Time to junk the concept of immediate gratification and whatever makes you happy, and talk to your Dad about a proper strategy for the future. Time to face the truth - time to shovel the gravel.
And if I were giving you fatherly advice, I would tell you to live a spartan existence, save your money, work on your career path, travel to overcome your ignorance, and don't get married. You're the sensitive Nice Guy type, it takes a special woman and this wasn't it.
And sure as shiat, don't go playing step-dad again.
Volenti non fit injuria