I don't know how, but my mom could open jars better than me when I was a foot taller and much stronger than her. She'd make me move furniture around, and I'd hand her pickle jars.
Doesn't help you now I'm sure, but I've found gently banging around the rim or lid of a jar on the concrete outside usually loosens the seal enough to open the jar.
(Outside on concrete because I don't want to dent anything inside.)
I find that it's easier to do left-handed, so that all of the torque runs through the fat part of the thumb, rather than trying to grip with that instead.