First Timer; Truly Confused

I've been dating an ADHD guy for about 3 years an I truly don't know how to improve it anymore. When I first met him, He's just like everyone's dream guy. He says the nicest things and is always very attentive. A month later, we started dating. All was good and nothing could possibly go wrong within a year's period. During the 2nd year, we started suffering some problems. He says he'll promise to call but he'll never call and start giving excuses. I'd let it go the 1st time but it went on and on and I got fed up. It got me thinking that he just didn't bother to make the effort or that he was seeing someone else or was thinking I was a boring person to talk to. Eventually, I tried not to get my hopes high when he said he'll call. I also didn't feel like I was his priority because he would cancel our date just to go out with his friends or he'll just cancel just because he "doesn't feel like going out". It was only during the end of our 2nd year dating that he revealed to me that he has ADHD. My perspective totally changed and just made me want to work harder to improve our relationship. Sometimes it works out and sometimes it doesn't. Most of the time I'm left confused and in tears because I have no one to turn to. None of my friends would understand what ADHD is even. My mother would understand but I don't want to add more things onto her plate as she herself is trying to handle my brother who is a special needs as well (Asperger's Syndrome). After reading Melissa Orlov's blog, she suggest joining a forum or a support group so here I am and I guess you guys could help out on how to communicate and improve my relationship!