Pffth, nah. I do the same freaking thing, so don't worry! Whatever you end up doing, you at least learned something, right?

Well actually no, don't answer that question because looking at that pixel-over so far, I automatically know that's a "yes." You got the curves of the tail and front leg especially nicely, but it looks like you're still kinda having trouble with angles and points, tbh. Looking at it up close, the ears aren't perfectly sloped / pointy / rigid like that. They round out a little, and even the points are rounded:

The reason I left a "jaggie" on that left (our left) ear there is because it looks more natural this way; if I were to have done it the standard way, it would end up looking too angular:

(Addendum: Uh, whoops, I forgot to fix the inside part of the left (our left) ear. hahah.)
I can cover that up later with shading, anyway! It's kinda a weird technique since it's an exception therefore it goes against what people teach you to do ("NO JAGGIES AT ALL!!!!"). It's a case-by-case basis, and it's hard to judge when it's appropriate. I'll just have to show you as I go along, I guess!

The lines around the stripes are also looking really crooked and jagged...but looking at the art, they're pretty rounded and uniform. The lines in this pixel-over are kinda going in all different directions (the top-left stripe is pointing down, the top-right is pointing up, etc). I'll let you try to figure out how to fix that one yourself. But if you get stumped I'll help out! :D

The points on the left (OUR left) whisker look pretty much fine, it's just the other set of whiskers that look funky. Again, it's just because the points are kinda going everywhere. I'd definitely use that left set of whiskers as a reference if you go and edit the right ones.

And this is more of a nitpick, but Litten's back leg should probably be curving in a little more. It's really subtle, but basically what your lines are doing right now is forming a straight line. Litten's back leg is subtly curved, like so! :D

As you can see all I did was literally change a few pixels. And the part of the tail closest to the fluff consists of one really long and straight diagonal line, which looks odd seeing how curvy its tail is. Curves are tricky!
Also, did you notice that I didn't get rid of that jagged line right in that area? I highlighted it in red for you. While it might not be the absolute, 100% best placement, THAT's exactly what I was talking about a couple of paragraphs ago. c: You got the right idea.

So far, so good! The front leg is looking great so far. I'd point out that its leg is more straight in the official art, but to be honest I like how you have it as it makes it appear to have more...depth? Realism? I dunno. I like how that looks, and it's not like it's technically incorrect or anything!

Can't wait to see the rest! What about the rest are you having trouble with? :o But yeah, the mistakes here aren't a couple really big ones; there's just a bunch of really small ones. That's not a bad thing! Actually that makes it easier for me, in a way! xD
@Sou