The ending of Donnie Darko.....?

I thought i followed the movie pretty closley and understood all of it but what happened at the end i still dont reall understand please some one give me a good answer.

Donnie essentially went back in time to save his girlfriend because he knew what was coming and he sacrificed his own life to save Gretchen, and ultimately the tangent universe.

Frank told Donnie all that would happen and it all hinged on the jet engine crash, so either Donnie stays in bed, dies and his girlfriend lives (he wouldn't have met her had he been in bed in the first place AND she wouldn't have died if she hadn't met HIM), OR he stays out of the house and lives but Gretchen dies.

Really, it's a love story where the protagonist sacrifices himself for his woman.

I think Donnie died in the earthquake.. not too sure either tbh

I think what it comes down to is he went back in time to save his mom and sister. There was that worm hole or whatever, and he had to go back to the plane crash day and sacrificed himself. It's been a while but I think that's the gist of it.

You really aren't meant to understand it. The entire point of the film is about perception of reality and at the end they try to get you to figure out just how you look at it.