Yes, sadly this is a quite common thing, partners not always wanting to constantly have to reassure someone and thinking it's too much work in the relationship. Honestly people like that just aren't the one for you then, some time you will find someone who can understand you better and you can both help eachother.

I find this to be a problem amongst normal people too. But with ADHD, we can hyper focus on what's going wrong and keep needing assurance. What's helped me is noticing everything that's going well in the relationship and letting my partner know about things that I notice. And then, I have my own entire life of doing things I like to do without my partner as well. I learned that loving myself was the key component to have a harmonious relationship. And focusing on things I like to do keeps me confident that no matter what my partner does, I don't have my entire world fall apart. Honestly, I've set up multiple "worlds" that are different from each other. My groups of friends showcase this - I have friends from my hiit gym, another set of friends from my ninja gym, another set from triathlon, another from climbing, another from running, another from obstacle course racing, etc. At the end of the day, your partner can't completely understand you since you think differently with having ADHD. It's your job to make sure you're happy. Your partner's job is to show you other possibilities and diversity of thought that you wouldn't otherwise thing about.