AITJ for dating my ex's friend?

(all names are fake obviously)

so for context my ex boyfriend, Jackson (19), and i (19) dated for roughly 4 months, we were both in the same friend group and just wanted to try it out. we called it off roughly 3 months ago.

after our breakup we remained in the same friend group and he had proceeded to date my (now ex) bestfriend at the time, hooking up behind my back and keeping it a secret from me. me and him have remained civil but both him and my ex bestfriend began talking trash about me to anyone who listened.

one of me and Jackson's mutual friends had introduced both of us to a boy named Mark after we had broken up.

Me and Mark clicked better than him and Jackson did but they are still relatively close.

In the past month me and Jackson have hung out 4/5's of it and we have even talked about dating as we both are interested in having a relationship with eachother.

So, i guess im asking if i would be the jerk for getting with Mark because he's friends with Jackson, despite Jackson getting with my now ex bestfriend.