Why do I have almost no water pressure after installing a new valve?
Likely that you have injected or dislodged some bits of stuff that have now flowed up and clogged your faucet.  Especially likely if you have one of those faucets with a single handle; otherwise how can you explain how the hot water is affected?  Those kinds of faucet have little channels that can be clogged with gunk; it happened to my shower.