The only way to learn how to sight read is to sight read. If you like Bartok, then "Mikrokosmos" is a great set of pieces to practice sight reading. They are in six books, starting with very easy (and also musically interesting, no small accomplishment) and going to fairly difficult. I would start with Book 1. If they seem very easy to you (you can play them at sight without any mistakes), then read through them all anyway. Then go on to the next one.
As for lost skills, with respect, I've found that my grownup students who say that they have lost skills really didn't have them to the degree that they thought they had them. (The "it's just like riding a bicycle" maxim applies to piano skills as well. You get a bit rusty, but you don't lose anything you can't get back with a month or three's hard practice.) It's just that the expectations change as you get older. Ask any four-year-old whether he can draw, and the answer will be an emphatic "Yes!" A 10-year-old, not so much.
Clair de Lune is a fairly advanced piece. If you can play it well, you have some pretty solid skills. But probably 95% of the people who play it play some of it well, and some of it badly, and their idea of "practicing" it is to go over and over the parts they can play well, and over and over the parts they screw up as well, screwing it up in the same way over and over again. And then, never really working on the end, because by the time they've done enough screwups, they go back to the beginning and assuage their irritation at all of their mistakes by going back over the parts they play well. (And then, 20 years later when they start taking lessons again, they play it for their teachers, warts and all, and say that they used to play it perfectly back when they were 15 or so. Well, no they didn't!)
Don't be one of those. Be one of the 5% who actually learn the whole piece. :)
Also, learn your scales and arpeggios. All the keys, four octaves. Start slow. Start with two octaves if you want to. But get to four. Start at MM 60, four notes per click (or MM 120, two notes per click if you prefer). Shoot for MM 120, four notes per click.