As far as pressure, the pumps you get with the kits aren't going to be high gpm pumps. I used a 4 gpm rv pump. I think the standard is a 2.5. So from what I gather, you have cold water flow and hot water flow but no in between. I usually turn the pump on then the cold all the way and then slowly and incrementally turn the hot up untill you get the desired temp. The valves are tricky especially with a lower flow pump. It just takes longer for the temps to change and when they do you've likely gone passed the sweet spot.

Once you have the temp set, leave it alone and dont touch it except to make very minor adjustments, but only on the hot side... even when it's off. Just make sure the valve on the showerhead is off and the switch is off.

I installed one for a guy here about a month ago and the hot side wouldn't flow at all. turns out the bottom side of his block had sand in it and had plugged the mixing valve. That cost him an extra hour of labor! Be sure your hoses are all clear.