[Mithalwen]

Beliriand was North West - the Ered Luin. Lindon the area to the west of the Ered Luin, realm of Gil Galad in the second age, was the bit of Beliriand that remained after the rest was flooded and broken in the War of Wrath. If you compare the SIlmarillion and Lotr maps you will be able to pick up the mountains curve and see the mountain that is the island Himling in the Third Age. I think only Beleriand was damaged.

I think that the world was originally flat but after the attempt to invade the Undying Lands by Ar Pharazon the world was made round. The Undying lands were removed from the circles of the world only to be reached via the "straight road"
by those who had the permission of the valar .... when they sailed instead of the ship following the curve of the earth it would keep going straight and so reach Valinor .. but I am sure that someone else can explain this better!!! The space on the round world vacated by ~Valinor was filled by the "New Lands" ie The New World - the Americas.