I definitely have the same problem (I just sent a work email, and have to go back to a task that's a bit difficult, so I'm just giving advice that I don't follow to a total stranger instead).

One thing I noticed about the "hard stuff" that I procrastinate away from is that I am a very slow thinker, and usually, the reason why I don't get into it straight away is that the first idea I have is not the right one, and I kind of know that in the back of my brain, and so letting the ideas flow freely in the brain while doing something else helps the good solution cristallize in a slow way. So when that's the case, I just need to embrace that and work on something else that's simpler like small frontend stuff that gives you good satisfaction for very little thinking.

Anyways, other things that sort of works for me are : lifting small weights while waiting, or doing grip training (I am climbing, so these are things that help for that), or doing dead hangs (I work from home...). Take a short walk. Like I literally used to walk back and forth to the bathroom when I was a kid.

I think that something like a underdesk bike, that's connected to a game on your screen would be kind of nice, you'd pause it while you work, and when you build something, you'd add an extra mile to that game... I don't know, I'm just making stuff up now.

Back to work ffs.