Its because your hands are in the same grip position over most of the day every day.
You need to take a bit of time over the day and do some hand stretching exercises especially
before starting and at the end of the working day.
Even trad the odd ground floor just to get your joints working and moving slightly different.
Yes getting a light pole is always a good advice but its only part of the solution.
I actually had to cut my window cleaning down to three days a week because I was getting horrendous
pain in my neck from looking up day in and out.
Even standing back as far from the property as possible didn't help It got to the stage last year when I
honestly thought I was going to quit.
I added on washing drives and grass cutting to make up the lost income.
Doing this has helped my neck pain a lot and its still improving which is a good thing.
Would rather be cleaning windows full time but needs must.
Wow that's awful, though good ideas about drives and lawns, hope it's working out ok. A brilliant piece of advice I got on here when I first started was to get as far away from the house as you can and look with your eyes not your neck and head, keep those as low as possible. Working so far but not always possible if things are in the way.
If I can't sort this trigger fingers then I could go part time and get another part time job, or temp job and clean windows around it. Or branch out. It's the repetition that's the problem with anything