You are not stupid, you got into college and found high school easy.

For me, I try to look at my overall goal of getting a doctorate. To make it easier, I set numerous sub-goals so that I have some motivation, some direction to go in. This can be anything from passing the next test to passing a year.

Also, you must prioritize and plan out your time. Ideally, make a schedule for yourself and give yourself extra time for each college-related activity. The reason for doing this is two-fold. First, it gives you more leniancy in finishing the task. Second, if you finish ahead of time, then you can feel good about that, feel good about finishing it earlier.

When working, take breaks. I can for some courses hyperfocus for hours on end, and although I have quite a good memory and intelligence, in my first year I wasn't using this intelligence enough. I was guzzling energy drinks like no tomorrow trying to study as though the sky would fall the very next day if I didn't finish. Later in that year and now, I realized my mistake and am far less aggrevated.

Try working with a study group, it can help reduce stress and tension in yourself, make it easier to learn by getting multiple views and you can teach others. That teaching helps you retain and apply the concepts a lot.

For me, when studying, I underline in my books first in pencil the general concepts. Then in blue or black pen I underline more specifics. If need be, a red pen. After, everything that still isn't sinking in and the very important concepts I write down only once and read them over and the book. When I write it down, I use my own words, not the book's wording so I can know and be able to apply it.

Also, and very important, give yourself rewards. These rewards can be coming onto here, getting ice cream, TV, etc... . You have to take the roles of the nagging parents and the student. You also have to minimize distractions, so study at home, campus, library, etc... but in as much peace and quiet as possible. Have some snacks and a drink nearby so you don't have to get up and run for some.