There are several things, an they all involve other people because it's their actions that make it very difficult to almost impossible for someone with ADHD to deal with it.
1: The people closest to me basically refuse to educate themselves about ADHD but have the audacity to tell me whats it like because they know someone who has it. They can't get it through their head that everyone is different.
2: The people closest to you still have the perception that you are the same lazy, unmotivated stupid person that "you were as a kid"
3: People who don't have it think it is just a fake, made up disorder so people like me can use it as an excuse for our actions.
4: People closest to you don't take anything you do or say seriously. Your always wrong.
5: We process things differently, I have never been fired from a job but I have had difficulties with co-workers because I have been able to my jobs in a way where my thought process has allowed me to be more efficient and along with my work ethic have made other workers look bad, not on purpose, just doing the job I was hired for. That has resulted in much resentment which was followed by work place bulling and false rumors about me to give me a bad reputation. I am a very big guy and I was an easy target for harrassment, if I stood up for myself a co worker would go to the boss claiming he felt intimidated by me. I'm in my early 50's and it still happens.
6: The isolation, the actions of other people, especially family and the ignorance and treatment caused me to basically isolate myself to the point that I am always alone. I do have a long time girlfriend and she knows I'm not the person that people assume that I am. At times she can be one of those as I don't talk about my ADD with her. It's a journey that I am alone on.
7: My siblings have excluded me in there childrens lives because of my difficulties as a child, my mother has not gone to bat for me as she talks AT me just like I am a child. 3 of my nephews are having the same issues I had but my siblings are in denial that they have ADHD, the mother does agree with me on that. They treat me like crap because my childhood, but watching their kids heading down the same path saddenes me. When I was kid ADHD was not known, todays it is and they just won't accept it that one of their kids has it.
The list goes on.