I'm dating my best friend (let's call them P.) and we're exclusive. P can be a slut when single but is loyal and committed when exclusive. There's this person (let's call them S.) who P had sex with before we were dating. That's okay, only S is still asking P to fuck daily. I know and have seen that P rejects S every time. Last night I even sent a voice message to S from P's phone as a reply to a fuck request, asking S stop bothering P/us but S replied with  stupid messages about P's stuff that he left at their place and if P had asked me if I wanted a threesome with the two of them yet, obviously trying to make me jealous.

I trust P. However I really, really don't like this S person and don't know how to handle the situation.
Last night I asked P why he was even still talking to S but they were avoidant and I think they said something along the lines of "we're friends"


I'm becoming the crazy obsessive possesive partner. I don't know how to handle this. I want to not be bothered by this but I am and I don't know how stop being bothered.