I can't help with any of the military stuff transition wise but - I can help a bit with the transition to school-life as an adult. I had several years where I was "working for the man" so to speak and then decided to go from college to university. I felt the same sort of 'regret' you did about not applying myself more in highschool. The plain, simple truth is ... most people don't apply themselves in highschool. They're too busy with their teenage-angst or hanging with their buds...
In fact, I considered myself lucky that I graduated college (which is further than anyone in my family has ever done) and was 'satisfied' with that. However, when I had a few years in of 'real-world', I realised I was capable of even more so I began to go to school part-time while working. Eventually, I went full-time as an 'older learner'. Sad when being 28 classifies you as an older learner, eh? Anyways, it felt weird at the beginning but if you work hard and take your schooling one class at a time, you'll get through it. Some classes are harder than others. You may even find yourself in a class that you had 'no-choice' to take just to find out it's a subject you adore and it could change the course of your whole degree! If anything, you may actually appreciate your university experiences now that you are older and will have a perspective that younger students don't have... you have the life experiences and the ability to see where your classroom experiences can help you in the real-world now.
If for some reason you can't get into the officer selection for this fall, don't stop there. Maybe take a course part-time and gradually work towards your degree. It took me eight years to do four years but in the end, it's a great feeling knowing you accomplished something you may never have thought you could.
Good luck with everything!