I've read your rent and I must say that I can recognize myself some years ago in your story. Studying without the ability to concentrate is hard, and even though while you are intelligent enough it seems like you keep on failing on your goals.
Yet, you are who you are and you're going to have to live with it. It is really easy to see your ADHD as a glass ceiling and blame your current failure as a student on it. Still it wont solve your issue.
Try to focus on the aspects of yourself that you can influence rather than the ADHD where you just will have to learn to live with.
A book that I would like you to recommend reading is called "the 7 habbits of highly effective people" by Stephen Covey. It was a real eye opener for me to focus on the right things and totally changed my negative attitude.