A lot of interesting responses, and I enjoyed reading them. Your problem was a one handed person that was weak as well.
IMPO the only viable choice would be a .38 special, 3-4 inch barrel, loaded with a light fast round like Winchester Silvertip 110gr. HP's. and practice A lot. I mean a 100 rounds a weekend for 6 weekends straight a lot.
I carry an old 44 mag loaded with extremely heavy HP's. It will take a mans shoulder off him "ONLY IF I HIT HIM". It does not matter the caliber or if its a semi-auto or not. If you cant hit them, your toast. If you can hit them, then 3-5 rounds center mass out of that .38 will stop them trust me.
I picked the combination I did for accuracy, weight of weapon (lack off), recoil (lack of), low cost of practice rounds, and ease of use.