I've been seeing this guy for about a month and it's going really, really well, and we've discussed starting a relationship in the new year (he actually asked me out a couple of weeks ago, but I said I wanted to take it slow).
The only thing is, a couple of months back I had a casual fling with my flatmate - we kind of used to get with each other out of boredom, and though I like him as a friend I definitely never had feelings for him, and I'm not even attracted to him anymore since meeting my new guy. We had sex a couple of times... and I caught chlamydia from him, which I didn't find out about until after I'd had sex with my new guy - so I had to tell him I was infected, which he was totally fine about, thankfully. He knows my 'number', that one was a serious relationship and the other was a rebound mistake.
He met me and the flatmate on the same night when we were at some party, so they're kinda friends as well. And obviously he comes round my flat. So I don't know if I should tell him that I had a fling with my flatmate...
On one hand I hate hate hate lying and feel like it's a bad omen to start a new relationship on lies, plus if he finds out somehow (I don't think my flatmate would ever tell him but who knows :/) he probably wouldn't be happy I kept it from him. And he's so understanding (take the chlamydia thing as an example) that I think maybe he'd be okay if I told him.
But on the other hand, the fling meant nothing to me, and I don't want nothing to ruin something so good. I don't want him to ever think there might still be something between me and the flatmate, like I don't want him to get jealous for no reason.
In a nutshell: meaningless fling with flatmate, met amazing guy and things are going really well, found out had STI from flatmate so had to tell new guy about STI, don't know whether to tell him about flatmate...
Have you done any of these?