In high school, every bump in the road feels catastrophic. When you get a less than satisfactory grade, it feels as though it could derail your dream of attending the college of your choice. When you find out your "friend group" has been trash talking you behind your back, it feels incredibly isolating and unrecoverable. When a teacher says she doesn't believe in you, it's so easy to lose confidence in yourself. It's a seemingly unending balancing act in which your very existence is defined by getting into that college.
When you see a graduate from your high school again over thanksgiving break, you notice immediately that there's been a change in them since you'd last seen them and you can't put your finger on it, but you can't deny it's there. They talk about how the world is so much bigger than your high school and you can only acknowledge it in a sort of distant sense as you're immersed in stress and near constant college talk. You can't imagine what it would feel like to be able to look back on high school in a past tense.
And then you start college and you immediately understand what your upperclassman had meant. Your immersion in college culture is immediate and complete. You can only remember high school in a remote sense and regard it with a mild grimace when you think of the SATs and can even begin to regard some aspects of your high school career fondly. Talking to your underclassmen only serves to emphasize how different your life is now, and you feel a distinct disconnect in where you were and where you are now.
You still have a massive workload and you're still sleep deprived to at least some extent. You still might have to think about how your application to graduate school should look and how you're going to craft it just so. You still have similar pressures to what you had in high school, and yet nothing feels even close to cataclysmic. There are still long nights, but they feel much less isolating. You stop feeling guilty about taking an hour to go to dinner with your friends and you stop feeling guilty taking time for yourself.
You have a sense of having arrived and you worry much less about how your application to school look and much more about how you can seek out every opportunity you can. You make friends with people in college that you never could have imagined being friends with in high school and realize your preconceived notions about people are wrong. You learn to fully appreciate everything you have now that you realize how lucky you are to have the resources you do.
Overall, the disconnect between high school and college stems from having a greater grasp of what your place is in the world and developing a greater kindness toward yourself.