Try making a "kitty burrito" - wrap her entirely in a towel, so her legs are bound in near her body, then just push the towel back to expose the matted area. With clippers, shave off the matts - they are probably tugging and bothering her already, making that area even more sensitive when you try to help by combing it out. The person holding the "burrito" can murmur comforting words, and try to keep some of her attention away from the indignity at the other end of the kitty.

Once the mats have been cut off, it'll look peculiar but be more comfortable for her, and the fur will grow back. Then you can work on her weight problem with the dreaded d-word "diet!"