I think learning EKG's and "pre-hospital" pharmacology (ie drugs for paramedics) as a basic is kind of a waste. It won't hurt, of course, but you won't have much/any opportunity to put that knowledge to use as an EMT-B.
I think it would be better to dive into learning anatomy and physiology. Make Anki flashcards as you go, so you don't forget the material.
To supplement that, go into depth learning about the various pathologies your patients have: What is CHF? What is COPD and why is it called that? What is "inflammation" exactly? Why do some patients need dialysis, how does it help them, what caused them to need it, etc? What, exactly, is cancer? What do all those lab values mean - what are abnormal values for K, BUN/creatinine, etc, and what does it mean for those to be too high/low? What issues could come from those abnormalities, and why/how?
There is so much you can learn that will be directly relevant to what you do as an EMT-B, and that you can apply right away in your assessments. EKG's are fun, but I think it would be more useful and relevant for you to understand the mechanisms that cause CHF patients to have edema in their ankles, for instance.
I would add... I don't think you're "over your head". Learning EKGs and pharmacology to the level that paramedics learn them in school is not hard. I just think it will benefit you more as an EMT and eventually as a med student/doctor to focus more on basic A+P and pathology, and building your assessment skills instead.
Also, don't buy a pharmacology book for medics, but learning about the drugs your patients take would be really really useful! Learning about how aspirin works, how lisinopril works and why it would be prescribed, etc, will really help you out a lot as an EMT. Learning ACLS drugs as an EMT-B would be mostly pointless.