I made hot and sweet mustard using mustard powder, and it’s too spicy to eat. How can I make the 2 jars that I’ve made milder?

I think you should continue to store most of the mustard as made, because that will last longer, more safely and taste better than modified sauces you can make with the mustard to dilute the intensity.
Then, as you need it and depending on how you like to use the mustard,, mix small amounts of it with mayo, butter, oil, or a relatively bland sauce.
Usage ideas:
1) mayo-mustard for devilled eggs, chou-croute or various sandwiches;
2) mustard parsley compound butter (made into roll, chilled, then sliced in discs) on steak or roast beef;
3) mustard-oil dressing on salads, especially potato or pasta salad;
4) mustard-bechamel on eggs or chicken.