Unfortunately there will always be people who don’t believe that what someone else experiences can be legitimate medical conditions. ADHD, in all its forms have been proven time and time again. Any medical professional who doesn’t believe it exists, either hasn’t seen the research or is choosing not to believe the abundance of evidence. Even in the medical profession there are those practitioners who make errors in judgement. It doesn’t make them bad people just incorrect in their analysis.
As for your mother, ask her to read “Driven to Distraction” by Edward Hallowell, M.D. In it he addresses those very issues and gives numerous examples of why those assumptions are incorrect. In fact, it would be a good book for both of you to read. There are many reasons that some parents refuse to (or take a long time to) accept an ADHD diagnosis. It doesn’t mean that they don’t believe you, or are not taking the issue seriously. For a parent there are a who lot of “Did I do something wrong?” type of emotions that they need to address as well as dealing with your actual diagnosis. She will most likely come around once she understands that it isn’t something she did wrong, or something she didn’t do.
In the mean time, keep talking about it and do as much research on the subject as you can. This is a perfect place to do some of that research.
Good luck, and keep your chin up.