There has been more than one time in my life where I had to ask myself if I should tell my SO that I slept with a friend/coworker, and most of the time I’ve elected not to mention it because it doesn’t matter and serves only to make my SO jealous.

I pretty much decided if SO is going to be around the friends in question I have a history with more than once or twice, then I would tell them so they don’t feel dumb being the only person in the room who doesn’t know. If Jake and Lena weren’t still a big part of your life I’d say you were fine but I would’ve said something sooner since they continued to be. Sounds like you understand that though and I’m sure not telling Sabrina was coming at least partially from a good place of not wanting to hurt her.

Of all relationship fuck ups this really isn’t bad, I think everyone makes a mistake like this at some point in their life. Sorry you’re feeling this way, but try not to be too hard on yourself.