My girlfriend of 3 years wants to breakup because I slept with a prostitute before we met...?

I was a teenager at the time and didn't meet her till 6 years later. She is really upset and says she can't look at me the same way. It was a one time incident forever ago and I've always felt bad about it but she is not being understanding. Long story short I've had a very promiscuous past while she has only had me. I understand her being upset. I'm curious how other women feel about this and how I should go about making her feel more at ease and be able to respect me even if she hat my past?