How can I prove to my girlfriend that the bad me is gone and is not coming back?

So my girlfriend has become kind of afraid of me lately because a couple of weeks ago I was put on this medication which was supposed to help me but instead just made me a complete asshole. So because of that medication I was turned into someone I completely wasn't and it made me do something I would never do which is threaten my girlfriend. I've told her so many times before that that I'd never hurt her and then I went off and said that. Now she's lost trust in me. I stopped taking my meds and I went back to my old self. And I told her that the bad me is gone and the good me is back and the bad me is never coming back, but she wants me to proof it. How can I?