Don’t just learn the formula. Learn what problem somebody invented it to solve.

A formula on its own is a set of instructions with the reasoning stripped out, so it has to be memorised and it tends to vanish under pressure. Somebody invented it to deal with an actual problem, and once you know what that problem was, the shape of the formula makes sense and can be partly rebuilt when you forget it. Understanding the question is what makes the answer stay put.

For the next formula you have to learn, find out what it was invented for and what people did before it. Work one example by hand without using it.