OK! EXCLUSIVE: Leighton Meester & Sebastian Stan Split

It seems that one of Gossip Girl's many real-life romances is over: Leighton Meester and her boyfriend (and sometime co-star) Sebastian Stan have split, a friend of the actress-musician tells OK!.

So why the split now, when things seemed to be going so well? Leighton called it quits, because she "doesn't know how to be in a relationship and doesn't really like them," explains the insider.

OK! GALLERY: LEIGHTON MEESTER'S A MEAN, GREEN, POWER-WALKING MACHINE

And since Leighton was the one doing the dumping, Sebastian isn't taking it too well, with our source saying he's "heartbroken" by Leighton's decision to end the relationship.

Leighton's reps have not responded to OK!'s request for comment yet.

OK! NEWS: LEIGHTON MEESTER'S "YOUR LOVE'S A DRUG" TRACK RELEASED