This sounds very close to what I went through.
I did good in school, but not as good as I could. Homework killed me every time. I skated through classes without worry but couldn't manage time or deadlines at all. I assumed it was something with me, and that people with ADHD were spastic and hyper like some of my friends who also were diagnosed.
I used to get extremely angry, and I was always sad, thinking that I was just a screw up whenever I failed to keep my temper, or whenever I missed a deadline or forgot to do something. Some bad stuff happened in my life as a result of that, I started going through depression.
Finally I went and saw a therapist, I explained how I felt about all these things in my life, how angry I would get, how sad I would get. I thought I had depression, or bipolar disorder or borderline personality disorder...then the doctor said something I didn't think any doctor would say.
"I don't think you have any kind of emotional disorder, because you should be feeling sad about your life"
That sounds awful doesn't it? I should be sad when I look at my own life? A doctor is telling me my life is that bad? But the thing was, it was true. My life was really sad, I had alienated friends with my temper, missed opportunities due to my memory, and wasted time on so many things I couldn't possibly keep track of them all.
That's when she explained that I might have ADHD. It was my eating habits that clued her in, whenever I wanted to work on stuff I cared about (animating, writing) I would binge eat all my favorite foods, and tons of caffeine. I did that at the time because I wanted "energy", binge eating sometimes gave me a scenes of focus and drive I rarely felt.
And of course it did! Caffeine is a stimulant! Food is fuel and your favorite foods are going to make you feel happiness and fill up with dopamine!
The problem was ADHD, I didn't have the focus to remember things long term, I didn't have the focus to regulate my emotions before an outburst, and I didn't have the capacity to think ahead.
Went to a psychiatrist, took a test, came back as a clear diagnoses. Started taking medication and I feel amazingly better. No more anger issues.
If you doubt it or don't understand it, watch this video. It is the most helpful thing I've ever seen on the subject of ADHD.