If you see your GP they may refer you personally for an assessment. If you get a diagnosis, it might make it easier for your kids to then get a diagnosis too.
Otherwise, I would see a private consultant as some of them specialize/favour it and would be more likely to give the diagnosis. You need only go for a while and you can prioritize it so it's not unattainable financially (I'm unable to work through disability and I did it using my disability payments.) I'm in my early 40s so they relied less on school stuff.
I got good results at school/uni most of the time, but performance was patchy and finishing A-levels and uni took me a few years longer than others as I kept having to drop in and out. Socially I always struggled (coming out with weird stuff; I also have autistic traits.)
People can do well at school with ADHD with a lot of parental encouragement and natural intelligence etc, but they will still have issues like teachers might think they're not putting enough effort in when they are trying, or the other issues you describe that can make their performance or relationship with teachers less good than it would otherwise be.
Once you have a private diagnosis, you should find it easier to get one on the NHS.
I didn't find medication that helpful on its own. On the NHS (I've got a diagnosis but am on the waiting list to start meds) they apparently give you lots of pointers to organize yourself at the same time as meds, which can make the results of both easier.
When it comes to tactics, you might find some better than others and be able to keep them up. You can do some of it without meds. I have a little notebook in my bag, where I write things I need to remember, and that helps.
Your feeling that you can't do anything about it might be due to your ADHD itself- low self esteem from experiences you've had, or the condition itself and its effect of mood dysregulation.
This is a book the NHS recommended to me- www.amazon.co.uk/gp/product/1641522720?tag=mumsnetforu03-21 even if you only take up one or two tactics out of the range described, it can help. If you've read this one before then try having another read, and take notes if you can.
Try not to be too hard on yourself. Let seeing yourself as having the condition help you forgive yourself when you struggle, as it helps you acknowledge you don't start on a level playing field to everyone else, and all you can do is try your best, and forgive yourself. Best wishes xxx