You’re not bad at math. You might have just missed one step in fourth grade that everything else stood on.

Mathematics depends unusually heavily on order, with each idea resting on one underneath it, so a single missing piece from years ago keeps collapsing whatever gets built on top. From the inside that feels exactly like being bad at the subject, and most people reach that conclusion rather than going to look for the gap. The conclusion does far more damage than the gap, which is usually small.

Work backwards from whatever is defeating you and ask what it assumes you already know. Keep going down until you hit something you are sure of, then start there and climb back up.