You have operated the Safety Pressure Relief Vale and they do not like to be disturbed. Once operated they tend to stick open and never seal back up again properly resulting in the discharge of you're central heating fluid. Ultumatley this will lead to low pressure shutdown and boiler failure. All you can do now is try to get it to shut it off properly by once again opening it up, and letting some fluid out. Fingers crossed if there is some grit or muck got stuck in the valve it may wash out and sit down correctly again and then you will have to re-presurise the system. If this still fails to work then you need to replace the PRV. The shower problem could be a few things it may not be the correct shower for the boiler or it is scaled up. The boiler may not even be powerfull enough for the shower. What boiler is by the way ?