I have extreme difficulty sailing upwind in 20 mph winds with my TI. I have tried furling my sail to all lenghts. I am sitting in the aft seat and I am the only person sailing the TI. My tramps are folded up on the amas. My sail is pulled fairly tight. Any help will be greatly appreciated.
You should be able to sail twenty with full sail, but it's all about your comfort. Let the sheet go a little, ie. loosen the sail some. You want a slight luff along the mast for best sail trim. It's also easier to learn sail trim by setting the sail and using the rudder to find the sweet spot than to trim the sail directly. When things are right, the boat will feel like it's taking off.
Read some books about sailing, you have a boomless, unstayed cat rig and most books are about bermuda rigs but the general info helps. Search the forum too, lots of great stuff written here, God knows I've become a better, faster sailor since doing my homework. Also, search the net for sailing forums on boats like the Nonsuch, which is an unstayed cat rigged mono hull with a wishbone boom. Lots of interesting articles available just by searching cat rig.
Good luck and happy sailing,