How do I convince my girlfriend I'm going to change?

This is a throwaway, clearly, since my girlfriend is a redditor as well.

I (M, 19) have been with my girlfriend (19) for over a year. I have just come to the realization that I am a compulsive liar, and many things I have told her were not true. It was a habit; I wish it hadn't happened, but it did. She found out about one thing, and I ended up telling her about everything else I had said that wasn't true. She no longer trusts anything I say or have said in the past year and wants to break up. She is the best thing that has ever happened to me and I can't lose her. She wants me to convince her that things are going to be different now, and I've said everything I can think of, but nothing seems to work. I have already started therapy to treat this and get better, I have started working on myself and moving my life forward. I want her to know that I really am worth her time and effort. What can I do/say to convince her that I have changed, I will never lie to her again, and she should stay with me? I know I sound like a complete asshole, and I am, I understand that, but I truly am trying to be different.

TL;DR: I have lied to my girlfriend countless times but have told her the truth about everything, how do I convince her that I'm changing and she should stay?