I guess I was a little depressed too, but I think I kind of felt it had to be this way to explain some of the time travel consequences. It was interesting to me that even though there was a "re-set", as explained in the book it wasn't complete. Sadie still had memories of Jake.
I guess they could of ended up together some way, but isn't it great how a good story can make you FEEL so much. I think that is the beauty in this. I'm with you all the way!