Thermostatic valves do need replaced from time to time. I tried cleaning one without success. Replaced it instead and the flow which had been dropping off slowly over the years before it fell off a cliff, went back to the previous performance.
I’d checked part number for the shower on-line and got a lot of prices. Weirdly though the next price was from the supplier who I had phoned to check something. They were 50% less than anyone else, so I bought 2 to have a spare for next time
As I understand it, as it ages, it lets less hot water through, so to keep temp constant, it lets through less cold water too. If it is the valve and you try only running cold water, the pressure should be fine