mixer tap broken no cold water

Problem with the shower: only very hot water comes out even when on a little and cold tap on full blast. The water is coming from a mixer tap attached to the shower. How can I get cold water out as well as hot so I dont burn myself?. The mixer tap works fine when running the bath, ie. cold tap and hot tap. When the central mixer lever is pulled up to transfer water to shower no cold comes out. Please help as I can't have a shower without scolding myself.

 

thanks Vicki.