Buddy, been there and done that. I will guarantee you this, you will meet another amazing girl and after a while you will realize the new one is the perfect fit.
You said you were dealing with a lot? Well, mentally you werent ready to settle down. You sometimes need to be selfish and work on making yourself better before you can totally commit to someone. I know how you feel and its not easy, you may feel guilty, lonely, and all the other bad shit that goes along with losing a girl you think was perfect for you. Whats bothing you as well is the fact that she moved on and you didnt. Dont let that obscure the fact that in reality, she probably wasnt the perfect fit for you. Trust me on this.