Water Pressure in shower?

[bigdog]

Having water pressure issue in bathroom shower? Also experiencing
hot/cold water flucuations when shower handle turned to hot?

Background....
I live in a condo complex. Condos water supplied by private wells with pumps.
Water pressure ok in all sinks and tubs.

Could my issue be the mixing valve to the water control handle?
If so, is this mixing valve easy to replace or do I need to hire a plumber for replacement.