First I'd like to say welcome. Finding joy and relaxation when playing is awesome. The only way I know how to break through a rough spot is by relentless practice over and over until you finally do.One method is to first get the fingering down then start out playing slow a few times through and then picking up the tempo a little at a time until you reach the correct speed. Hope that helps...
As I've also returned to piano after a long time, I can relate to you, although I think you haven't been away quite as long. My goal also it to be able to play Chopin and some other favorites of mine. But most of his works are way too hard now, I can play through one predule, but it still needs work to sound really good.I decided to take the long and slow way, started lessons and started playing the easier pieces that I can download from this site. I will work my way through the easier pieces first to make sure I have the basic skills and experience before really starting on the difficult ones that I ultimately want to play. It has been a real pleasure to see how fast I am learning now, even though my memory and concentration needs to get better. I can't remember much about playing as a kid, but I certainly remember advancing much slower.Of course I could try to learn one difficult piece with a lot of work, but I want to be able to play more than just one or two pieces properly. So it might take me 5 years or 10 years to be where I want to be. But practicing is really relaxing and enjoyable (especially now that I have a really nice piano) so what's the hurry. I'm playing for my own enjoyment, I'm too old to become a pianist anyway Good luck!
I think, as far as I can say, you might want to learn more about tonalities, the circle of fifths, key signatures and the fundamental building stones of music, like triads, chords, harmony and scales. Why? Your potential struggle with "Clair de lune" stems in all probability from a lack of experience at all those basic elements. There you might need to catch up a bit. Learn all you can about intervals, triads, scales, learn the circle of fifths, maybe even a bit of counterpoint and harmony.Then learn the easier Chopin Preludes like op. 28 nos. 4, 6, 15, 20 and some "easier" nocturnes like op 9, nos. 1 and 2, op 37,1 or similar ones. And don't forget op. 48,2, the sibling! I love that one as much as no.1 and it's very very precious, but much easier! All of those are as well very much worth the effort!