I can't give you any help on eventual health problems, but I can tell you about my experience.
I definitely experienced it during my first year on technology college. I remember a specific subject in math that I just couldn't grasp when I first started - and it really hit me hard, because I had gotten used to be great without any effort. I never really had to study when I was younger, and I think that was the actual problem - reality struck me and I had to do an actual effort to succeed. At this point it wasn't possible to understand everything in an instant.
The first year wasn't interesting at all, but I soon got used to it, became way more focused on the 2nd year and finally excelled on the 3rd year. Even if I did have it in me, I couldn't take advantage of it, because I experienced challenges I hadn't faced before.
You asked how we get back up and how we manage to stay focused. Hmm, I don't actually know how I got back up from the discouragement - it must've come naturally as I got more focused. One thing that probably helped me was to remember what subjects I was going to finish soon and which ones I was going to continue with for another year or two. The subjects finishing soon would require more attention, but I wouldn't get so stressed if I didn't grasp something in a long-term subject - because I'd have plenty of time to catch up.