I was really good at taking tests too. What you’re running into is the fact that ability amongst children varies greatly, often so great that school can feel like a cakewalk for them.
At some point school gets hard. You just haven’t gotten to that point, and may drop out or go to the wrong college like I did and have it never happen.
If and when it does get hard, your lack of study habits will make it pretty much impossible for you to succeed. I don’t know of a good way to avoid this. Maybe try to go into business instead. Much harder than school, but you’ll probably be better off.