HELP- Communication break down with ADHD partner

I'm new to this forum - my partner and I were never married. We both have liked each other for 5 years and finally started dating seriously about 6 months ago. I have anxiety and my partner has ADHD. The first 5 months he was hyper focused on the relationship - he quickly told me he loved me and always made time for me. Then about a month ago there was a sudden shift - we barely saw each other and it seemed like he had time for everything else but me. I didn't fully understand the symptoms of ADHD, and when he shifted his focus from our relationship to his other interests I felt left out and excluded. I became very sad, our communication broke down and by the time we finally talked about it he ended the relationship. I handled the situation poorly by withdrawing affection and decreasing communication. When we finally talked about it he told me that he loved me and thought I was amazing, but he thought we were a "good" fit not the "right" fit because "things should be easy for the first year of the relationship"... he kept focusing on how neither one of us should have to "change".

I want to rebuild our relationship, but am unsure of how to communicate with him. He has really rigid thinking ("if I"m not 100%, I'm 0% - I shouldn't be dating if I'm not 100%"). He was diagnosed with ADHD within the last year, and briefly saw a counselor - but he didn't' like the way medication made him feel and decided that he couldn't afford counseling services. So right now his ADHD is untreated and I don't think he realizes how his symptoms are affecting his relationships. How can I approach him in a way that makes him feel heard and understood while still addressing how his ADHD symptoms (and my anxiety symptoms) impacted our relationship? I'm concerned he thinks communicating differently means that we are "changing" for each other (something he views very negatively).

Has anyone else had a similar situation? Or had to address this type of "all-or-nothing" thinking/refusal to "change" with your partner?