I have gradually lost water pressure in my shower. My water now comes out of my showerhead and not out of the faucet when I turn the shower faucet on. Is my problem in the shower control knob maybe clogged or needing a new stem?

Your mixing valve and diverter are likely clogging up. I'd disassemble and clean or even replace the cartidge. If it a Moen you can get a free replacement by calling Moen customer service.

Ballengerb gave you great advice. Start here and let us know what you find