An IQ which only places me in the top two percentile of all Americans cannot be enough to fulfill the intellectual requirements of a physicist or chemist, is it? Someone give me some hope.
Nonsense. In the first place, while IQ is indeed a pretty good measure of intellectual potential, it doesn't measure all of one's capacity for thought, conceptiualization and calculation. In the second, there are lots of very successful chemists, physicists and engineers out there with IQs less than 132. Finally measured IQ does indeed change over time, depending on how one uses his mind. This is particularly true at the extreme ends of the scale in the top and bottom deciles.
Some folks are very quick learners and solvers of problems (and thus have strong measured IQs ) but lack the insightful conceptual ability and the ability to integrate ideas from several complementary areas over the long haul. Others are slower at the start but get better over time with continued study and learning. Very often these folks end the journey with better overall knowledge and understanding - a tortoise & hare thing. Said another way, some consistently do well in pop quizzes but flounder in comprehensive exams & vice versa.
Hope and persistence are things you will have to find within yourself. Get started.