I found an old shirt from a girl my bf used to see

that by itself means nothing. Unless it was found in recent dirty clothes

Get rid of it or Forget about it. It's not important unless it's used daily....

I still have like two shirts of ex boyfriends, sometimes i see Them i think oh, i should throw Them but then i forget and do other things.. i mean of course my boyfriend is not so much noticing this like us girls i think, but could be the same for your boyfriend like me that he doesnt even pay attention to it.. but indeed depends, if was Just wore is different but if is old...

Ask your boyfriend to destroy it, may be he forgot he still has it.
If he doesn't he still feel attached to that relationship and he has to break it up.

It's just a shirt. Years ago, I was on a break from my long-time gf, and started seeing a pretty, but, wrong side of tracks type woman. She wore a lot of sweaters, big poofy sweaters, and one left tons of puff balls all over my apartment. Seemed like no matter how much I cleaned up, I found more.

When I got back with my gf, of course, she found many in my apartment and asked about them. I told her they were the landlord's daughter's, who'd sometimes bring cookies and baked stuff to the apartments (which was true), and she bought it. Said oh, those things get everywhere, and that was it. I was finding those little stupid sweater poofs until I moved out.

it is only a shirt

Pee on it!

Just kidding, ask him what's up with him hanging onto an old shirt of hers.