You're doing something that you really want to do, and that's the biggest part of it. That's probably the way it is for most SPs. If you're motivated by something, you'll keep at it. And putting in the study time is important.
One mistake I made early on in university was putting too much effort into some projects that I enjoyed and not enough to others that were worth more. So I think it's good to keep a sense of priorities. Know which classes are important, which assignments are worth a lot. And then make sure you do a good job on those ones.
Try to find co-op terms and hands on practical projects/courses. You'll ace those. And they will make the readings and coursework more meaningful.
I'm not organized either, but there are some terrific tools lately that can make even us SPs look pretty good.

If you sign up for gmail, it comes with an online calendar. Mark down your due dates as you get them, and then there's little chance of forgetting. For exams, study a bit at a time. Review each day, and then there's nothing to plan for. That's the way we learn best anyway. It goes back to the whole "living in the moment". "Study in the moment" too.
Take note of your character, are you a morning person, or evening? Don't take 8AM classes if you are a night owl. Do you study best in larger blocks, or a little bit at a time?
But most of all, be confident. ISFPs are very smart. I'm sure you'll do fine if you work at it.
