why do I have high water pressure in the bathtub but not from the shower head? already changed plumbing and changed the shower head
Take a gallon bucket and measure how long it takes to fill the bucket from the tub faucet, the shower head, and the shower pipe without the head on it. If the time to fill is relatively close between filling with the tub faucet and the headless shower, then it is probably the shower head. The shower head you buy must not allow more than 2.5 gallons per minute in the US, many are below 2.0 GPM. There are ways around it, such as modifying your shower head or putting in more than one shower head.
I believe you will find almost all shower heads have flow restrictiors in them to save water. In the past I have removed these or modified them to provide the flow I wanted.
My three faucet diverter valve had an old dome style washer jammed into the shower pipe in the valve seat. It took me a while to figure out what was going on. I used an ice pick and piece of wire to remove it. Now pressure is better than before.