[MF] My coworker (24F) cheated on her BF with me (25M)

This is my first story here and it will probably be long, so please be patient… also, English is not my first language.
So, a bit of introduction first: I am a 25M sound engineer working in a musical (you’ll see why this matters). I have an open relationship with a really cute girl -let’s call her Sara- who is 3 years younger than me, very sexy, and not relevant to this story (sorry hahah).
Anyway, I’d been working in this musical for months, and after some ups and downs, I decided to quit because I needed some fresh air. Then, out of the blue, the week before leaving the job I started noticing the costume designer (we’ll call her Paula. She is 24, lives with her 43yo BF of 6 years). Not particularly because of her looks (black, loose work clothing doesn’t really let you notice anything) but because I was suddenly noticing a cute-but-mischiveous side to her that I didn’t know. So we started talking more (as much as the backstage of a musical will let you) and soon I found myself following her around and noticing that she also gave me a lot of attention. As it was my last week in the job, I thought “fuck it” and went all in.
2 days before my last day I started giving her a massage and waited to see her reaction… well, apparently she loved it because 30min later she was giving me one and kept coming back to where I was.