Shower - Rain head won't turn off so hand held can be used solo.

Shower heads are relatively inexpensive compared to the thermostatic mixer units!

The Best Showerhead for 2022 ' Reviews by Wirecutter

https://www.nytimes.com > Home > Bathroom
Apr 8, 2021 - But if your bathroom meets the criteria, and you're the type who goes to the gym primarily to sit in the steam room, this luxury pick is the ...
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Best shower heads of 2022 ' CNN Underscored

https://www.cnn.com > 2021/01/08 > best-shower-heads
Jan 8, 2021 - Other shower heads we tested · Glacier Bay Modern 1-Spray 8-Inch Single Wall-Mount Fixed Rain Shower Head (starting at $29.98; · RainSpa 3-in-1 ...

The 5 best showerheads we tested in 2022 - Insider

https://www.insider.com > home > best-shower-head
Jan 4, 2022 - The 5 best showerheads in 2022 ; Best showerhead overall: Kohler Flipside, $59.29 from Amazon ; Best high-pressure showerhead: Hopopro High- ...

remove the small filter in the hose in the head or at the wall

Pull it apart and see if the flow restrictor is plugged in the water inlet to the shower head.

This is the car and truck section, but you might try youtube for videos.

you can replace head or you can try soaking it in bleach see if that works first.

Disassemble and soak it in a lime cleaning solution or vinegar. I have include a link below with more information for you. I hope this will solve your problem.

http://www.wikihow.com/Clean-a-Heavily-Clogged-Up-Shower-Head

well you can do a 3 way shower valve , or a mixing valve with 2 control valves. or you can do just to regular shower valves one for each of the heads you want one for the rain and one for the shower head. this way is cheeper and you can have one on hot water and the other cold water or the other way around or both the same temp.

Rain head showers are a nice thought, but they come with a bundle of problems. This is common one, though, so fear not.

As long as your regular shower head isn't losing pressure (I'm assuming that you have at least two heads instead of just one rain head) I would recommend carefully removing the head with an adjustable wrench and letting it soak in CLR (calcium lime rust) for at least an hour. Often times minerals in water will build up in the small holes on a shower head and greatly reduce efficiency.

Remember to apply teflon tape to the threads before carefully retightening the shower head to avoid spraying water from the wrong end!

Most handheld showers use a hose to connect from the existing shower arm down to the hand held option.  Some units come with attachment points on the shower arm, but I believe that you are looking for a unit that has a hook that you can mount where ever is comfortable for you.