Nope, I'm not getting old and struggling to pee.

I recently fitted a new mixer shower as the old one kept dripping. Since then there are times when it is almost impossible to turn on properly. It will turn on a little but only about half flow and then it is almost impossible to rotate further.

With water supply off the dial will rotate freely all the way. It seems to be worse if the hot water has been on.

Any thoughts?

Is it fed from a combi or other high pressure system?

If so try to fit a mini expansion vessel (shock arrestor) in the supply pipework as it could be excess pressure locking the valve up.

If you have a combi' boiler or your hot water cylinder is "unvented" (hot water @ mains pressure) the i'd suspect a low pressure mixer's been used as opposed to a "high pressure one" tell me more.
I may be nipping aht for a quick ride if it doesn't rain too heavy so response may be slow