Sorry to hear of your troubles. I have a dear friend who is going thru the same thing - she broke her wrist after slipping on ice. Needed surgery, had a plate and 7 screws implanted. She's currently going through PT and they told her it could be as long as 2 years before she's fully recovered. She's feeling somewhat depressed about her prognosis and keeps offering to sell her longarm to me. I told her it wasn't time for that yet, although I will come over and quilt some stuff on it for her, if it makes her feel better. I still have hope that she will recover full use of her arm, even if she doesn't. <3
I have done this, so it might work for you:
Use clamps. You might end up needing to use a couple different cutting mats and rulers, but you can do a lot of straight cuts if you clamp both ends of the ruler. You might also ask around your friends and guild to see if there is an Accuquilt or some other kind of fabric cutting system available for you to use temporarily.