My ADHD Fiance Broke off our engagement

My fiance and I have been together for 5 years.  Two years ago, we entered into a long distance relationship (about 1600 miles apart) while he is attending graduate school.  6 months in, he came back home and proposed to me.  I was ecstatic.  About 6 months after that, I started to notice signs of ADHD in him, though he was 30 and had never been diagnosed.  He was struggling in school, not producing the required work, and was worried about getting kicked out of his program.  His communication with me, especially the intense, "I miss you so much" type of feelings that permeated our first year apart, started to wane, and then to nearly disappear.  For days on end we'd communicate only via text, and sometimes he'd leave me hanging for hours at a time.  I convinced him to go his psychiatrist, and he was diagnosed with ADHD and placed on medication.  I was very supportive of his diagnosis and helping him with coping mechanisms.  During this year, he got a DWI and promised to stop drinking, but he didn't, and he lied about it for weeks before I found out.  I forgave him, and even told him the drinking was ok as long as I knew about it and it stayed contained.  A month later, he broke off our engagement over the phone, two days before I was supposed to fly down to see him and spend the summer with him.  I am devastated, and wondering if this behavior is typical in ADHD relationships?  He has still not given me any answers as to why he suddenly broke off our engagement, and I feel the ADHD probably has a starring role in the blame.  Has anyone else experienced this?  Did he lose interest due to lack of constant stimulation?