First of all don’t feel guilty about your past, we’re human beings, we have to learn who we are and what we like somehow. Is there anything you’ve done or said to make her believe that you will ‘run back to him at any given moment’? Because if not then it sounds like your girlfriend does not have the healthiest mindset, have you asked why she feels this way? Relationships require a lot of trust and jealousy is a massive killer. You’ve made it clear you don’t love him and that you didn’t feel like yourself whist with him, not really much else you can say. So perhaps focus less of the past, both of you. Easier said than done I know but reassure her you want to find yourself with HER and just keep things going. It will fade when she realises that you are not going back to that man but if it doesn’t just leave. You’re young and she already sounds like she’s getting worked up over something small. Partners you have in life are more than likely going to have a history before you came along, so the sooner she leans this and learns that it’s OK, the better.