how to repair a shower/bath mixer?

I have had a drip coming from my bath and shower mixer for a while now, but we have lived with it. But now when I press the lever to have a shower the water still comes out through the bath, it also comes out boiling so if I have a shower I burn my feet. I call a plumber and told me to replace it outright. I dont want to replace it. Does any one know what I could do to fix it??