And I know, the levels aren't really hard compared to what the last few chapters have to offer. It's just that Sanctuary took me 40 minutes (I'm not kidding) to upload because I couldn't complete it. I guess SMB isn't really the best thing to play when you're feeling tired.
You know, maybe the fact that it took you 40 minutes to beat your own level isn't an indication of good difficulty. Wouldn't that be a sign that something is wrong with your level?
As for the levels, they look pretty good for 4-bit maps. Sparkle is a bit harder than "easy" as air control isn't always easy to master and the last jump can be tricky. Horseshoe is simple enough and belongs in Easy, but Sanctuary wasn't good at all. You just repeated the same obstacle endlessly and thus there's no reason to push forward and try to beat it since the player knows there will only be more of the same, you have to vary your stuff a bit.