The water pressure in my upstairs shower is drastically different from hot to cold with hardly any pressure on the hot side. I have a single handled valve.

It sounds like you have problem with your faucet (Cartridge)

Can you tell us the brand and the model number of your faucet, is any plumbing work done lately, did this just happened or you had the problem from long time? Thanks.

John

I'm with afaroo on this, sounds like a cartridge or valve body issue,. Also, can you tell us if you notice any difference AT ALL between hot and cold at other fixtures in the home.. Lee.