I think this is the sort of problem that a teacher can help a lot with. I was playing similar repertoire and had similarly stiff wrists. A good teacher helped me start my technique from scratch, and after a year and a half of working with her I feel much more relaxed, in the wrists especially, and I produce a much better sound. I don't think I could have made similar progress just by reading people's accounts of how you should play; too much face to face feedback was required for me to "get it."