Perhaps the activity just doesn't interest you or doesn't suit you?

I've been repeatedly frustrated by efforts to learn new languages, and usually let my attempts fade off. Without a clear motivation for learning the language other than to learn it it's hard to maintain the effort. (This has changed recently with my attempt to relearn Spanish, I was almost fluent 18 years ago and now have a good reason to regain and maintain it.)

So what sort of novel activities have left you feeling tired?

I used to be the same way - it eventually turned out I had ADHD-PI. Now that I take d-methamphetamine that's no longer a problem, fortunately.

That's called depression.