Stephen and Pap, hit it right on. Motivation and finding a good study routine (whether it is study group, a stack of flash cards, etc.) are the keys to success going through API. Is the material all that difficult, no, it isn't. The degree you have is not going to help or hinder you, even in the case of Aero Engineering degrees, they were told to "forget" all the other stuff they learned and take Aero 1 and 2 from the stance that it was a "basic/intro" class. It all boils down to the "firehose effect", as you have heard many times, the material is coming at such a quick rate, and you are held accountable, through the tests, of knowing that material cold. It is all done to see who can prioritize as Stephen mentioned, and buckle down and study those 4-5 hours a night, then study sessions on the weekend. My family basically "gave up" on me, and went on vacation to visit other family and friends while I went through API. I personally never "really" had to study hard in college, CIS major, the occasional crash study session, or late night programming, but nothing sustained. So, here you are in API having to make new study habits, it is quite the shock.
As Pap mentioned, the process doesn't end, you go through API, thankfully you get a break between then and Primary, but it starts all over again in Primary, but this time you memorizing systems, course rules, emergency procedures, power sources for gauges and instruments, radio calls, preflight items, and the list goes on and on. Each item builds on the next, and you will go over it many times in tests, briefs with your Onwing, and in the plane. There is ZERO opportunity to gloss anything over, you either know it or you don't. Granted you can miss stuff, or not have a complete grasp of something, and they will teach it to you, but there are certain things that you just HAVE to know, from the emergency procedures to course rules that will get your butt handed to you if you don't "regurgitate" them.
Here I am, more than two years from my Winging, and I am still studying my butt off. It just doesn't stop. Nowhere near the intensity of ground school and primary, but there is always something you need to be studying. Whether it is your quarterly NATOPS open and closed test, yearly instrument ground school, yearly instrument and NATOPS check flight, pilot upgrading syllabus flights, PQS, studying tactics and weapons, etc.... All while handling a ground job.
Sounds pretty ****ty, doesn't it? Well, I will tell you, that flying is one of the most amazing jobs you could ever have, and all the pain and overcoming hurdles is worth every hour of pain studying to get where you want to go.