If you got your braces tightened today, and you can't eat anything, then mashed potatoes would be something that is really soft, and they'll definitely get you full. My daughter has had braces for 2 years, and she still gets them tightened once a month, and she knows what you're going through. She suggests that you eat before you go to have your braces tightened. She's suppose to be getting hers off soon.
Anything like mashed potatoes, pasta, soup, pudding, etc. are all good things to try..also your orthodontist may have suggested that you could take ibuprofen to help with the discomfort..it works wonders for during ortho treatments!
The best thing to eat after a tightening that becomes painful is oatmeal. Cream of wheat also. It's filling and comes in different flavors. Anything you will have to chew including pasta will cause discomfort.