Far too young to have to be dealing with this

My boyfriend and I have been together for four years (he's nineteen; I'm eighteen.) Last year, he was diagnosed with ADD, and all of the sudden, things started to make more sense. For a while, there was a kind of honeymoon period where I could very happily excuse his behavior, thinking, “he’s on medication now—things are going to get better.” Surprise—things haven’t gotten all that much better, and this past weekend, I got to the point where I strongly considered going after him with a large mallet.
This past Sunday, he was supposed to come over to my house. He had been away for a week, so I was really looking forward to seeing him. A couple of hours before he was supposed to arrive, he sent me an e-mail saying that ‘something came up’, and he wasn’t going to be able to make it. ‘Something coming up’ is what he always says when he cancels plans, and no matter how many times I tell him that I want an actual reason, he’ll never give me one. I later found out from a friend of mine that he went over to one of his friend's houses for an impromptu D&D session.  Okay. There are so many things wrong here-- chiefly, he promised me that he would see me; I don’t understand how a) that would mean less to him than his little troll figurines, and b) why he can’t realize that it means something to me. My friend also told me that oh, by the way, he was leaving the country for a week on Monday. When I confronted him about all of this, he had no explanation, literally saying either nothing, or “I don’t know” when I asked him what he could possibly have been thinking. He didn’t seem to understand why I was so upset, nor did he seem to care.
I know that it’s bad form to tell someone with ADD how disappointed you are with them, and how much you wish they would put in more effort, but honestly, if he hasn’t gotten the message through four years of me very patiently reminding him that being in a relationship involves actual commitment, I don’t know what else to do.
Sometimes, I feel like I’m dating two separate people.  There's the one who I really fell in love with, and then there’s the one who forgets to show up, forgets my birthday, doesn’t care about my emotional state, and doesn’t seem at all motivated to fix things. For so long, I have been giving him the benefit of the doubt—maybe he does care; maybe he is trying; maybe something really serious happened and he had no choice but to cancel our plans. But now, after this, I feel so lonely, and so betrayed.  He wants us to stay together through (and, it is insinuated, beyond) college.  But until he realizes that he can't keep treating me this way, I don't see that happening.
I guess my questions are these:
1. How can he be so self-centered? Does it really not occur to him that I might be hurt by his actions?
2. How can I get him to understand that I can’t be the only one putting some effort into this relationship?
3. If and when this happens again, how should I handle it without driving myself crazy and making him feel worthless?

Please help; none of this falls into the category of normal high school relationship angst.