Reincarnation

[Celebuial]

I know this has been mentioned in other places, but I don't think it has really been discussed in-depth (sorry if it has, but I couldn't find it!)

I'm confused.... Is Glorfindel in LOTR the Glorfindel from The Sil? Surely if he was he wouldn't have just been left behind to help guard Imladris when the Nine Walkers set out? More to the point:

The whole idea of reincarnation in Arda confuses me. I have only read the first two volumes of HoME so far, so bear with me, but I can't seem to find much that makes sense without being contradictory. I haven't read the letters either, so you may be able to see why I'm so confused.

At first I thought that reincarnation wasn't possible in Arda. Then I thought it was possible for Elves who have been slain or who have died of grief. Then I read the Story of Beren and Luthien.

For about three years now I've been completly flumoxed by the whole idea. I think it's a shme that I don't at all understand this concept as I think it's one of the really intersting little things that you can find in Tolkien's work.

I would really appreciate it if one of you omniscient Tolkien experts could shed some light on this for me: I'm worried that if I don't understand it sometime, I may end up loosing sleep, and I'm certainly missing out!