Instant Pectin is for making a "no cook" freezer type jam and is destroyed by cooking. Reprocessing with regular pectin probably won't work unless you never added sugar the first time and add it the second time in exactly the steps called for in the recipe. No guarantees it would still gel however.
Other options, besides just tossing everything and chalking it up to lesson learned, you could add Instant ClearJel® to individual jars to thicken up the solution; or you could find Pomona Pectin
and reprocess your 'jam' using it. This commercial pectin is so good it will gel water. Of course your fruit that has been cooked and canned twice may not have much left in it to be worth eating.