I don't think you're selfish. It sounds to me like what you just are is lonely, and that you wish your boyfriend had more time for you than his life allows. It's okay to have those feelings and to feel sad about that.
It sounds to me like you two might be able to find some middle ground here by making sure that you are making time for some hanging out on the regular, assuming he has that time (he just may not, but we'll get to that in a sec). For instance, can you two find a couple of hours at least once a week that is only for the two of you to see each other? Additionally, what about you also joining in on one of the extracurriculars he is part of so that one of those things is something you can both share and do more together (shared interests are always beneficial to relationships, anyway)? How about you switch that text time to phone time? Or figure out a way to hang out together most days for a half hour before school? Or have lunch at school if your schedules work together like that? These are the kinds of suggestions I'd make to him, and then ask him to help think of more like this with me, were I in your shoes.
But probably before I did that, I would ask for a time to sit down and talk, and open that talk by letting him know that you still simply need more time with him than he is making available. I would suggest you start by saying that you at least need that one couple-hour-spot a week, as a place to start. I would then ask him if he really has time in his life for a dating relationship, and if he currently doesn't, if he really wants to make the time for one. If he does, he'll obviously need to cut out something else. If he doesn't, then as hard as all this is, and as much as it will suck and hurt, I would suggest you move on. If he just really values everything else in his life more than this, or wants other things more than he wants a romantic relationship, nothing is going to change, and you're just going to feel endlessly awful trying to get someone to give you time who doesn't have it to give or doesn't want to give it.
What do you think about all that?