Dealing with Stiffness

hey,
I'm about 4 weeks post arthroscopy and I'm finding that my knee is still very stiff. I can't bend it very far and it requires a lot of concentrated effort to straighten and bend it. I still can't bend it enough to go up and down stairs or even walk 'normally.' I've been doing the home exercises and stretches that my physiotherapist gave me but I find that whatever range I gain while doing the exercises never lasts long and I usually stiffen up again an hour later.

I'm a little worried because I am having another knee surgery in about a month and my OS says I have to get the knee moving before they can do the next surgery.

Any suggestions on how to overcome stiffness.