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.

One huge impairment that your

One huge impairment that your boyfriend still has is adolescence. You are 18 now and as a woman you are growing out of it and for the first time you are starting to step to the side and observe everything from a different perspective. You are starting to notice other people, other people's needs. You are not as controlled by emotions. Boys are a little slower and without being a professional or someone who knows more than what you have told us in here it sounds like your boyfriend mostly suffers from being a teenager. Maybe you have grown apart?

If my ADD comes in between me and something scheduled then I wouldn't contact the person I am supposed to be with or supposed to see. Because I wouldn't know. I would remember in the middle of the new activity and by then the former would have already contacted me asking where I am. He chose D&D over you. Simple as that.

1. How can he be so self-centered? Does it really not occur to him that I might be hurt by his actions?
Most likely not. Being able to understand that other people are hurt by your actions is a grown up feeling. When children are able to step into someone else's shoes they are usually described as "empathic" etc. It is usually acknowledged.

2. How can I get him to understand that I can’t be the only one putting some effort into this relationship?
You can make him aware of that that is how you feel. If he doesn't realize and try to change from there then he isn't ready to understand that.

3. If and when this happens again, how should I handle it without driving myself crazy and making him feel worthless?
If your boyfriend plans to leave the country again without notifying you I don't think you should be with him. It may sound harsh but.. I know life is busy and distracting but forgetting to tell your loved one about something as exciting as a trip abroad that makes no sense to me.

I think I sound harsh in this post and I apologize for that but based on how I interpret your post I think it's just wrong to blame it on ADD and lead you on. Try to evaluate your relationship. How did it look one year ago? Has he really changed? If you think about it? Or is it the way you see things that are different now?