I just finished grad school - that led to an awesome career in business. I liked school, but that was because I focused on the classes and learning methods I enjoyed (case studies, debates, theory, etc.) and bull****** through the rest. ENTPs can do that very well (although in engineering??)
Things we are good at: generating ideas, brainstorming, pros/cons lists, and finding better ways to do things. Things we are bad at: focusing, listening to lectures (shoot me), and keeping organized/ not procrastinating. The only way I survived was by becoming organized ("organized chaos"), and making studying and doing assignments into a game. I made it a point to innovate my study habits after each test to try to figure out what worked best for me (hint: you are probably one of the few "P" types in your class - hence you need to study COMPLETELY different than them).
I figured out that I am bad at studying for hours and days before a test. It actually just stressed me out way more and it seemed the longer I spent studying, the worse I did. In my last year of grad school, I would "study" by basically setting up and organizing my notes and thinking about the "big picture" of what the test was going to be on, and all the main concepts, chapters, what I grasped/ didn't understand, etc. Then I would wake up the morning of the test and study my organized notes (in a quiet space with NO distractions, even phone) for 3 hours. I aced every one of my tests that year. This was of course just my optimal method, but wanted to share my years of trial and error haha.
You said that you stopped playing sports. I too, stopped playing competitive sports in grad school. My school work, social life, relationships, and pretty much entire life was turned upside down. My depression that I always had fully manifested itself and the two years after I stopped playing sports were the worst years ever. My health also majorly suffered, and I gained (and now lost and am in better shape than ever) about 60 pounds. Why? Because as ENTPs we have TONS of energy, and especially social energy, that we need an outlet for. Sports is such a mental "break" for us - or else our thoughts just drive us crazy.
Sports also helps you stay organized. I didn't realize how much having a scheduled practice made me organize the rest of my day. When you don't have that anymore, you go crazy because you procrastinate with everything! Make sure you do something active - even if it is a gym class or rec sport. It helps all other areas of your life! Especially school! Even if it is waking up 30 min earlier and walking outside - its going to help you focus more. I wish someone close to me had encouraged that more when I stopped playing sports.
I am still working with organization in my life. I have come to the conclusion that I probably will never be the routined person I want to be - but the good thing is, there are careers MEANT for people like us - where our adaptability and lack or routine is actually best (consulting - any industry - look into it!). While I might not be able to organize my entire life all at once (yes, I have made master plans a la Ben Frank, such an ENTP thing to do), I have learned I can make little additions (and subtractions) at a time. Work in exercise, standardize the way you take notes or study for a test, keep everything in an organized folder (this was big for me), keep a calendar on you at all times, and set up study times or coffee dates with people just because you know it will force you to work - even if it is just out of guilt.
Lastly, hone your people skills. Us ENTPs can manipulate people (I think) more than any other type. Watch how other individuals work, how they communicate, who takes charge and how, etc. Learn how to get people to like you. Ask to help them and also compliment them and ask for help (these were big for me as I am very independent, it doesn't come naturally). In the end, in whatever job you have, people skills prevail.
Hope this helps! If you have any other school-related questions I would be happy to chat!