I dropped out of my maths degree after struggling for six years (!) at university - I had fallen too far behind. I can't just blame my part-time jobs for that. Especially early on I often dismissed problems and assignments that I felt I wouldn‘t be able to tackle in terms of time or intellect. This wasn‘t what the successful students did. They took on every problem with determination and focus. No matter how hopeless…
I had this problem and noticed it among others too. I think part of the problem is that in High School (and many times in undergrad, at least the beginning) you could get away with not doing the homework/studying/etc. So while you have the intelligence for the material you never learned the work ethic associated with achieving at the higher levels (and studying didn't force the material into long term memory as well). Sometimes I wonder how things would be different if these people learned an (academic[0]) work ethic early on.
[0] I stress academic here because at least in my case I was working a lot during undergrad. 40+hrs/week while in JC and 30+ when I transferred to a uni. But succeeding in those areas aren't the same as academic work ethic and I do want to acknowledge people that just ran out of energy.