I’m allergic to lemons, limes, and all citrus fruit. what can I substitute in making my own jams/jellies, other items that seem to require the juice of a lemon?
If you are not allergic to citric acid you can use it as a substitute for lemon juice in canning.
It looks like the primary purpose of the lemon juice in preserves, jam, or jelly is to increase acidity for safety.
So you should then be able to substitute any other acid to serve this purpose. The other common and versatile acid that comes to mind from cooking would be vinegar. It looks like vinegar can be substituted. I've seen instructions to use about half as much white vinegar as lemon juice. That last link also mentions wine as a possible substitute.
Since the other result of changing lemon juice for plain white vinegar would be less flavor, you may want to try to add some flavor back in. You could use a more flavorful vinegar - apple cider vinegar is apparently preferred for this reason, even though I think it'd take more of it. Or you could try adding some other flavor in to sort of balance the jam/jelly. Something with sour notes, or bitter ones, or just something different and contrasting to make the flavors pop. I'd think something like cranberries for the sour, or pine needles (as a tea in various ways or as infused sugar or syrup) for the citrusy aroma - or just some other juice or syrup you might have on hand to give a little something to your recipe.