I have always found self-diagnosing as idiotic, but honestly, this is 100% me. Can't concentrate on anything, hesitate and procrastinate a lot, also am forgetful. Never had major problems though, always have had good grades for whatever reason. Should I seek help or try to keep it under control myself? I am almost out of school and I feel relieved; so much pressure when you try to concentrate on the topic, but just physically can't. I feel much more comfortable learning the topics I personally like.
There are plenty people with ADHD-PI and good grades. Many goes undiagnosed into adult life as well.
Seek help. Be prepared to have to explain it a few times. Some doctors are amazingly reluctant to accept that ADHD doesn't mean stupid, lazy or crazy. (Yes, there is a book by about that name. Can't vouch for it yet.)
Also, medicine or not, be aware that you might have a few advantages: When everyone else freeze (exam, first to an accident etc) if you are prepared you can think clearly when few others can. You most likely can study very well on your own (but going to class is still recommended just to have an idea of what the school/teacher/professor thinks is important.)
Try to use your special abilities like: thinking outside the box (automate things), eye for details (something is fishy), people skills etc. I'd also like to think that a job were you are clearly ON or OFF would be an advantage.
You might find a 40 hr/week job easier or harder than college in terms of concentration. In any case, there is a clinical procedure for diagnosing types of ADHD if you want to know for sure.
Would you say this describes you?