"Medical Terminology" is pretty much a language of its own. You don't so much memorize it as learn the basics and go from there, much like learning any language. Much of "medical" stuff is based in Latin and as you learn, you'll find things just get easier. One place to start is by taking an A&P course. That will both introduce you to the language more properly and you'll also start learning MORE about the human body that will eventually allow you to begin to think well beyond EMT-level stuff.
You could also take a "Medical Terminology" course but often that results in a review of A&P simply because you're using the words to describe things that you find in the body. Those courses are NOT easy even if you've had a full year of A&P and are that much more difficult if you haven't.
Pay attention to things, write things down to look them up later, and get used to using the terms. Eventually they'll become part of your "normal" language when dealing with other "medical" folks. Much of my early education in patient care required a goodly amount of contact with physicians (specifically Orthopedic Surgeons) so I had to learn their language so that we both are speaking a common language to describe an athlete's injuries. Trust me, learning the language didn't happen overnight. I've been in healthcare, learning and speaking the language for about 20 years. I'm still learning and I expect to keep learning the language until I no longer work in any healthcare area.