Am I being Immature and irrational for behaving this way?

Personally I don't think you're being immature or irrational. I think she's kind of stupid for going back to the guy who made her lose faith in future relationships. I wouldn't get down on yourself about any of it, I'm sure she cared for you while you two were together *meeting the parents is always a big step* but she obviously has a need to be cared for by the man who hurt her. Sometimes it's a stupid thing we women do. It's an irrational need to be loved by those who have hurt us in the past or simply a need to be wanted and accepted. Her issue is that after he cheated, she lost a bit of faith in herself, she probably often wondered what was wrong with her, why she wasn't good enough or how she messed up. For him to take her back, that said to her that she was good enough, that nothing was wrong with her. In reality though, he's a jerk and your behaviour is completely acceptable. You don't need something like that bringing you down so I suggest you do your best to move on however, don't feel like all your time was wasted, smile because I'm sure you have many great memories from the experience. Life's a lesson and it's up to us to decide how we'll learn.