Here are the two methods I’ve used for years, depending on the type jar I’m opening:
For store bought jars, like pickles, jelly, etc. I just use the end of a butter knife and slide it in right underneath the edge of the lid and lift outward, and this pops the seal, and you can open it right up.

For home canned items where the outside part that screws down over the top seal (you know, the two part system for home canning), If the outside part has become rusty looking or stuck because of food or whatever, I run hot water over it for a couple of minutes, then try to open it by hand, then try tapping the part that is screwed to break loose any rust/food and see if it opens, then if this fails, I use the butterknife method. On the top seal, the hot water loosens the rubber gasket seal, and you can also use the butterknife to pop that seal if it doesn’t come off by hand.

These methods have never once failed me. Happy eating!

Open it with your left hand.

I used to have trouble with about half the sealed jars (and some already-opened ones) I tried to open. Now, I NEVER have problems.

Yes, it is as simple as that. I am right handed, and it is stronger than my left (my overall arm strength is nothing special). But believe me, it works. When you open it left handed, you are pulling your arm BACK INWARDS towards your body, where it wants to be. Your muscles there are stronger than the ones on your right arm pulling your elbow OUT AWAY from your body.

Also, the issue of having a good grip goes away. When you have sufficient arm power (which you get instantly by simply using your left), you will find that your ordinary grip will be fine.