Back in November, I started actively dating for the first time in my life and I was fortunate enough to have some success quickly. How do I re-engage old intimate partners?

Case 1) I had a ONS with this cute girl from school. She seemed to like me and I made her laugh a lot. I was inexperienced enough to not know to keep it going. I agree with the adage on here, "a ONS is just a failed plate." She had the right amount of non-committal vibe and sex appeal that I feel like an idiot for not keeping it going. I only texted her once 10 days later as per recommended on this sub. How do I re-engage?

Case 2) Got drinks with a friend from school, we came back to my place after and hooked up. It seemed like it was going to continue regularly, then she found out I had a girlfriend in an open LDR, the friend told me she was down for casual sex but didn't want things to get messy so it stopped. Anyway now the girlfriend thing is no longer happening. This friend and I have decent rapport, we've messaged a few times since. How do I re-engage and/or communicate I no longer have a girlfriend?

Overall) Throwing in some additional superfluous information. I graduated shortly after both events, been traveling, now I'm back in the area, I live 1 hour away from school, but living at home with the 'rents. They are both at school.