I agree, I love Memento.
I think Teddy was telling the truth. Leonard is Sammy (not literally, of course), but Lenny's story is actually his that they do have similar memory problems, but he cloaks it as Sammy's since he felt guilty (or is in denial about) being the one who actually murdered his wife by injecting her with insulin. Leonard seems to have this problem of believing what he wants to be true or remember, hence the missing pages in his file. In the first few minutes, after killing Teddy, he destroys the Polaroid of a dead Teddy. It's basically his only objective in life: looking for his wife's killer is all he has, once he does find out the killer, there's really no purpose for him to do. He'll eventually find another John G to do the same.
Of course, I haven't seen this movie in awhile, so...