It must be terribly frustrating to know what you want to do academically but feel so helpless to do it. You know you are smart enough to do well in school but something keeps getting in your way. I’m not sure if the problem is ADHD. What you describe could be accounted for by any number of learning disabilities as well as by an impulse control disorder. You absolutely need to get an accurate diagnosis.
Most colleges these days have a counseling service available for their students. You can get some testing there to help you understand just what the problem is. Your parents don’t have to know about it unless you want them to. Once you have been evaluated, you and the counselors will then have a solid basis for developing a plan to help you manage school. It’s also possible that you will need some accomodations to help you do your very best.
There are lots and lots of smart kids who have learning differences that make it difficult for them to be successful in school. Being able to sit quietly for long periods of time, to change subjects at arbitrary times, to focus on several different subjects every day, to take timed tests, etc. may be the traditional demands of American schools but they are not necessarily reflective of someone’s intelligence or abilities. Let’s face it. You’ll never have to do things like that once you get out into the work world. Even teachers get to walk around and to change things up a little when they get restless.
I hope you will take steps to learn more about your own learning style and how to use it. Then I also hope you will stick with your goal of becoming a teacher. Too often kids with undiagnosed learning differences decide they are stupid and develop terrible self-esteem. We need more educators who understand that being different doesn’t mean being deficient or dumb. It just means they need to work a little differently. Good minds like yours (and your future students) shouldn’t go to waste.
I wish you well.