The replacement knob should be made stronger (or much cheaper!!!) as it's obviously a design flaw.
The showerdoc reply,
Unfortunately the manufacturer dont make a more robust control knob for that shower. They say you need to be gentle when switching the control knob off and do not force it.
When fitting the knob please remove the cover and push the knob on to the cover, failing to do so will cause the fins to be pushed out on the rear of the knob and crack them. This causes the knob to break after several time of use.