You're 23 - only 7 years out of compulsory education. But yes, were probably handicapped by the paucity of an education system teaching to the test, than teaching for understanding of the subject. That is deeply regrettable, but not your fault - the whole community of STEM professions is seeing this problem (and, for all I know, other fields as well.)
I was sorting out my technical college notes from the 1980s last week - came across a load of notes in my own handwriting on the use of Laplace transforms to solve 2nd order differential equations. They scared me!
Very seriously - if you're struggling at this point, you really do need to go back to the books and refresh GCSE maths, or you'll just keep struggling. Don't just look for patches on specific points - get yourself properly up to speed on the subject generally (ditto basic physics), you'll thank yourself later for the effort.
G