Eee! You're up to the angst. xD;; There's a lot of that from here on out...;;;;

Okay. Basically, the Syaoran who has been travelling around with them is a clone Syaoran, the real Syaoran being the one trapped in FWR's hideaway place.
Clone!Syaoran was created by FWR basically to collect all of the feathers.
Okay, Real!Syaoran gave Clone!Syaoran half of his heart in attempt so that Clone!Syaoran might develop one of his own and thus not go crazy when Real!Syaoran was awakened. The seal of the heart is contained within Syaoran's blind eye.
So, when Real!Syaoran is awakened, the seal on Clone!Syaoran (our protagonist up to this point) 's eye broke and the half of the heart that originally belonged to Real!Syaoran returned to him, making Clone!Syaoran turn into the heartless ruthless-&-determined-to-collect-every-feather clone that he was meant to be. Which is why he has no feelings anymore and continues without his friends. He was designed simply to collect those feathers no matter what and that's exactly what he's going to do now.

I hope that made sense to you? :x