Have exactly same issue. I use a stopwatch on my watch, set it for 45 minutes, and every time it goes off, I force myself to do some short useful activity. This may be:

- a short stretch,

- a short light weight lifting session,

- making a coffee,

- cleaning up a small part of the house,

- doing a small step of any other physical chore.

Those can take 10 seconds or 5-10 minutes, depending on the activity. Usually there will be 1-2 longer breaks of this kind, and the rest are the shorter ones.

I especially like how some house chores goes away thanks to this!