Donnie darko fans??????????????????????????…

i have a few questions. first of all what is the significance of sharita (the asian girl) she seemed to be important to the movie but i couldnt figure out why. and my other question is how did donnie darko travel back in time at the end? he was with gretchens body and the sky was opening up as a portal right? did he somehow get into the portal or what? and the whole thing with the jet engine falling off of the plane and going through the wormhole/portal, that scene at the end confuses me a lot.