There should be a spring loaded pin within the temperature control knob. This pin is meant to butt up against the white plastic cam (your second picture) when the temperature is about 38degrees. You can then press a button in to exceed this temperature. I would guess that you've replaced the knob in a different position so that its always in the "hot" zone. Take it off again, line up the stop pin to the right of the cam stop, and re-fit it, with the button depressed. When fitted, release the knob and it should fail to rotate anti-clockwise past the set point unless you depress the button again.
If you've moved the white plastic cam, you'll have to re-calibrate the shower. Take off knob and cam, run shower and with knob loosely replaced, set temperature to 38 (or whatever is comfortable), refit the cam so that the knob, when fitted as above, points to whatever marking (red dot or similar) is on the body. Then refit as above without disturbing the cam.
