Responding to an add on craigslist personals?

Friday out of an impulse I responded to a personals add on craigslist looking for couples. I have NOT done that before. I have been in a relationship for 4 years. I knew she had my email password and she found the email I sent. Naturally she is devastated. I can try to explain it all kinds of ways but the fact is I was looking without permission. How do I regain her trust. I love her and want her but she says this is beyond forgivness