I’m sorry that’s the way things have turned out for you, but I also would be very hesitant to excuse that kind of inconsiderate, hurtful behaviour as “symptoms of the ADHD mind”. ADHD is not a reason to tell one person you love them despite not making time to see them for three weeks, and looking for other relationships. I don’t think this person deserves (or wants) the consideration and effort you’re putting in to understand him. No matter what challenges a person has, they should still be held accountable for their behaviour and responsibility to others. I think he was trying to get off the hook of having a serious relationship with you, and using ADHD as an excuse. The fact that he told you he “doesn’t know how to have a relationship”, but yet is signed up for a bunch of dating sites, should tell you a lot about what he’s looking for. Being hard on himself is just typical victim behaviour so that you can’t say anything about how hurtful and inconsiderate he’s being to you – the real victim!
I hope you find someone truly nice and don’t settle for bad behaviour just because someone claims to have a diagnosis to explain it. Invest that love into yourself instead!