Your momma didn't raise no boz. You let her control situation when your instinct was telling you that she didn't want to commit. What did you honestly expect? Never ask that question in this scenario unless you're prepared to walk. Instead of asking that sour puss question, be the man and confront your suspicion. Explain that get a sense that she may not be ready for a relationship and follow with an affirmation that that's okay (this will keep her from feeling intimidated so she can be honest). Then discuss from there (but keep your cards close to you). If an open relationship is what's best, then do that and tell her that you will treat your time together as if she's the only one and that you expect the same. The best way to get someone to commit is to make them feel like they are free.
I think everyone already hit the points with regards to investment - chalk it up to experience.