gretchen was his GF.
great that you brought this up LM. i agree with both Murrin and sarge that now ive been all the way through the web site, seen the directors version of what happened and watched the movie again im left somewhat dis- satisfied. the paradox and what caused it are left largely unexplained and some symbology that on first viewing seems relevant to the plot is actually just there to confuse you.
what caused the paradox is deliberately left vague imo for what reason im not sure. where did the sequence of events start and why? a black hole was created and donny needed to block it. to do this he used the artifact(jet engine). so the paradox started with the creation of the black hole right?
but how and why was the black hole created in the first place?. until i know or someone explains to me how the paradox in time started the plot will seem a little smug and insular to me. the only explanation i can seem to find is that there was a glitch in the universe! whats a glitch? everything else is so well thought out except what started the whole thing.
having watched it again last night im more convinced than ever that this is a film about god. i said in an earlier post that his english teacher uses the phrase 'deus ex machina' meaning machine of the gods which is what donny is. when donny explains that he can see the 'future worms' eminating from peoples chests his science teacher asks "if we can see our own destiny then we can choose. there would be no such thing as fate".
donny replies "what if we choose to follow gods path". thats what he does, he follows gods path to right the paradox and save the world. thats why he dies happy. hes reached enlightenment. i think its a film about devine intervention.
EDIT: thought about it some more and this is what i think:
Donnie in the primary universe always gets crushed by the engine. At the beginning of the movie, when we hear Frank telling Donnie to get out of bed, that took place in the TANGENT universe, not the Primary! Remember before that scene occured, we got a black screen that said 'October 2, 1988', well, that was October 2 in the tangent universe. We don't actually get to see October 2 in the Primary universe until the very end.
See, thats why Frank told Donnie that he had to save him. Remember though, Frank didn't save the Primary Universe Donnie, only the Tangent Donnie. This was important because if Tangent Donnie died as well as Primary Donnie, then the tangent universe would have gone onto October 30th without Donnie, the wormhole would have appeared and destroyed both. Tangent Donnie HAD to survive in order to be there on the Tangent 30th to send the artifact back to the Primary universe.
so basically tangent donny is on an eternal loop saving th world and as long as hes doing that the primary universe carries on as normal.
and a complete explanation of Deus Ex Machina: literally, machine of the gods--- a person or thing that appears or is introduced unexpectedly and suddenly into a situation and provides an artificial or contrived solution to an apparently insolvable diffuculty---- in other words, the gods cheat

It'd take you a long time to blow up or shoot all the sheep in this country, but one diseased banana...could kill 'em all.
I didn't even know sheep ate bananas.