Why doesn't she trust me? I slept with with my fb the day after I met my gf I did it because I wasn't sure if she wanted

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But she thought we were already in a relationship after the first day we met and kissed.
I kept it my mistake a secret for one year, until she found out, one year ago.
I regret cheating so much, and she still doesn't trust me at all.
The thought of losing her tears me up!

Should this be the end?
Telling her I didn't know we were in a relationship doesnt work, she still says I cheated.

But there's more:
While keeping it a secret, I still stayed normal best friends with my fb.
She holds that against me too, even though I broke all contact after she found out.

What should I do if reassuring her I won't cheat now doesn't work?