*Hug* Because you said you needed one, which I can understand. My boyfriend was a mechanical engineering major, and he spent day and night studying while he was in school - it was very demanding and at times put a huge damper on our time together. So I want you to know that I can definitely identify with some of the loneliness you are experiencing right now.
THAT BEING SAID...
It sounds like you are taking this very, very personally, in a way that she probably doesn't mean for you to take it. She isn't rejecting you, as I think you said you felt earlier. She is doing the best she can to get through something very demanding. This has nothing to do with you, period, so you really don't need to feel bad about it (easier said than done, I know).
A lot of people are suggesting to spend time studying together. That works for some couples, but I never got much done during our study dates, so I wasn't a fan. Since my boyfriend and I are still dating and not living together or anything, I prefer to see each other only once or twice a week and not have that kind of pressure hanging over my head (spend time/study, spend time/study - that is one thing I absolutely cannot multi-task at, and maybe you are both the same way). Even if we just go out to dinner for a couple of hours, or if I go over to his house to spend the night after a 12-hour shift as a CNA, that's enough for me right now. Back when he was in school, we had a fairly similar arrangement, and it worked beautifully. I don't know if that approach might work for you, but it's something to consider. (Also: if you are living together or plan to live together in the future, especially if she's still in school, obviously you will need to learn how to be around each other without actually constantly BEING together, know what I mean? But that doesn't seem as important right now as just spending some quality time while allowing each other space to get through school successfully. That part can and definitely should come later.)
Final word of advice: when things got tough during my boyfriend's schooling, and when I started to feel very lonely, I always pushed myself to do one special thing for him that I hadn't done in a while. Make a special dinner or dessert, draw up a homemade card, write a letter, do a chore he needs to get done - anything I knew he'd really appreciate. I know you do that too, but the key is in the attitude. You cannot tell yourself that everything will get better because "she'll see how much I love her and make *me* the priority." Acting without expectations and giving freely is the only way this relationship will really survive - whether you're in school or not.