Had anal a few months ago and can't stop thinking about it

A few months ago I gave in to my sexual fantasies and had sex with a guy off craigslist. He had a ten to twelve inch penis that deeply penetrated my virgin anus. At first it was painful, but then felt amazing as his twelve inch penis thrusted into my ass.I consider myself straight and have a girlfriend and have had sex with her, but sometimes I can't stop thinking about the guy from craigslist. I fantasize about him penetrating me again and bringing me to orgasm. I've considered meeting up with him again. What should I do?