Low water flow and temperature from bathroom faucet

Hi, my bathroom's hot water tap is only pouring out a little bit of barely warm water. The rest of the house's faucets work for both hot and cold water. The bathroom also works for cold water.
I did some of my own testing & found out that it's not the tap, the metal hose that connects to the tap, or even the valve that stops water from flowing.
Which means, I know the problem isn't in the actual bathroom, but I don't know what the problem is.
Could anyone please give me some advice for how to fix this? It would be very appreciated. Thank you in advance

 

Did you remove the hot side faucet cartridge? It could be clogged up with debri. You didn't quite say it but did you remove the water line at the stop valve on run the water from it. Did it have a full flow?

 

Why don't you turn the hot angle stop off, disconnect the flex hot water supply line at the faucet, aim it into a pail and turn the angle stop on - see if you get good pressure.
If your pressure is good, the faucet is at fault.