This is interesting to me. I find my own behavior is the exact opposite; I have no internal concept of need. I will do everything in my power to avoid starting a task until someone says it "needs" to be done. So in my experience the need is always externally motivated: it's usually just a deadline, or some other authority micro-managing me. ("I'm meeting with [customer] tomorrow and would like to show them [x] from project [y] is that ready?" => I will move hell and earth to do [x], and not a minute sooner. There's some activation threshold there. It is definitely not calibrated where it needs to be, but my ability to "thrive under pressure" has allowed me to cope, as long as someone/something holds my feet to the fire.)
Also I find deadlines are a lot more effective for adult-me than they were for child-me. I think that's mostly because I've got project managers to lean on who are creating those schedules, tracking them, and keeping them visible. Whereas in school child-me was expected to be my own project manager. (Yeah, that was never gonna happen. Many tried to get me to jot dates down in my little day planner. Many failed.)
Disclosure: I'm not clinically diagnosed with ADHD, but from all I've heard/read about it I would not be terribly surprised to find out I have it. (Although I would be surprised to find myself having sufficient motivation to go out and get proper help. ;-P)
If you are a woman and struggle with anxiety, it might actually "be" caused by ADHD or autism and struggles around those, and there is some play going the other direction too. If you are experiencing mental struggles that is "medication resistant", consider seeking a more thorough psych eval.
In my case: I am near physically incapable of doing things that I "need" to do, with the more I "need" to do something, the harder by brain and body will rebel against doing it. For most of my life this has mostly resulted in me being a very unregimented student who struggled with large projects and time management. I was lucky to have one of my main interests being "learning" so I basically independently excelled in school, so nobody even thought to ask for me to be tested. I finally reached my personal mental/autodidactic limits in college, and only made it through with luck and a good support network and privilege.
The hardest part for me is getting treatment. One of the things I "need" to do is find a psychiatrist who can fully diagnose me and prescribe me something. My doctor is, rather unhelpful, or doesn't realize she needs to help me. Either way, that struggle plus the state of undersupply in psychiatric workers, plus the fact that pretty much no pharmacy in the southern half of my state has any supply of any ADHD meds means that even if I could pull myself together to get that help, I couldn't get any medicine anyway.
The disability part is terrifying. I will never be "fixed". Roughly only 2% of the population is ADHD, and the zeitgeist in most of America, no matter how mildly progressive we get on things like gay marriage, continues to be "if you aren't perfectly productive in the system, you are lazy, bad, and deserve suffering". I don't expect anyone to ever legislate employers to just deal with ADHD employees, so I figure I'm going to get fired eventually. If I wasn't into books and then reading things on the internet when I was 10 as a form of escapism from home life, I would not have lucked into a lucrative job as a programmer, and already would likely be homeless. I've only ever been told I'm extremely, and obviously technically excellent, but because I struggle with time management and struggle to gain access to the medicine I need, society's opinion is that people like me aren't worth the trouble.