How can I win her trust back? Is there hope?

Okay, I'm not sure this is the place to ask but right now I really need help getting through this. I met this girl 2 and half years ago. We met, we talked, we fell in love la da da can't talk about that right now. However, aproximately nine months ago I messed up and I closed my eyes and pretended her best friend was her. I really didn't want her best friend I just wanted attention or something like that , now she knows and she don't want to talk to me again. Since then I have asked her to marry me and she has said yes but now thinks the last nine months of our life were all a sham and all lies. I admit I screwed up and I was stupid I took the advice from her best friend who told me I shouldn't tell when I wanted to tell her right away. Some girls or some guys with this experience can you please tell me some tips on how to deal with this, what to do, and is there anything I can do that will make her trust me, cuz I love her so much and I messed up and really regret it.