Is there any way to re-use old yarn?

I was knitting some mittens but I wasn’t satisfied with how it was turning out, so I’m having to take it apart now.
I was wondering if there was any way to re-use the yarn, because it’s all thin and gross looking at the moment. I’m already bummed out about taking the whole thing apart, and so the idea of letting it all go to waste is extremely unappealing.
What can I do?

What kind of yarn is it? If you unwind and wrap loosely it might help. Often I’ve just ripped and reknit so it might be fine.

Most of the time, you can just reuse it as is and the item will fluff up when you wash and dry it afterwards.

If you want to, you can fluff the yarn back up by winding it into a loose hank (maybe wrapping it aroud a large book, which is what I usually do.) Tie it in three or four places with waste yarn or string, then dampen it and, if it’s non-felting wool or acrylic, throw it in your dryer.

If it’s yarn that can felt, just barely dampen it, shake it around and dry it with a hair dryer on the low setting. I usually don’t even do that unless the yarn is really dirty and smelly, because when you felt projects that takes care of a multitude of yarn sins.