Hire a prostitute to bring to a bar and pretend she's your girlfriend, that way you can run the same cheating-on-the-gf game as before. If that's what you were doing before.

Either that or if you were planning ahead there are plenty of other girls' numbers stashed away in your phone from when you had the gf but couldn't act on em. Call em up and be like,

"I found your number in my phone, I don't remember how it got there, did we fuck? Want to do it again?"