Okay so when ever I go to take a shower the valve that turns the water on and off comes out and water shoots everywhere. the first time i had to turn the main water off put the valve back in and turn the water to off then went and turned the main water back on. I have got to where I can here the pressure change and I catch the valve and turn it off before it comes completely out. Needless to say, my showers are not the most enjoyable thing right now, and trying to keep the valve in place while trying to wash is nearly impossible. I am frustrated and just don't know what to do. The water pressure is very good---could it be too strong? I don't know if it's bad installation or a bad design. PLEASE HELP......
Here are pictures of my shower faucet:
Here are pictures of my shower faucet:

