Friends and my hectic schedule

I always try and keep in touch with my friends during the school semester but it gets hard. Being a nursing student is hard, being a nursing student with a lung disease is ridiculously hard. I am almost done and my friends and I have all stayed in contact but lately one friend has been saying that she feels like she tries all the time and that I do not make an effort. What everyone has to understand is that - I don't go out on the weekends, I work. During the week I go to school and am exhausted by the time I get home. I often have reading and studying to do which takes up the remainder of my night. Sometimes I just sleep because I feel like crap and sometimes sleep is the only thing that helps. On the weekends when I work - I sleep late. I literally set the alarm to give myself enough time to get up, do my therapies, eat a little something and shower for work. If I have to wake up earlier, I feel it. Once I get tired and over exert myself I get sick. The only time I usually see my friends are on Saturday nights because I bartend so they come to drink and see me. Other than that - I hardly ever see anyone. I even tried to make a girls night with Dawn and Danielle on Tuesdays just for an hour or two to watch One Tree Hill and catch up with each other, but I happened to be placed in a 12 hour clinical on Wednesday which means I have to wake up at about 4am - so obviously no more girls nights. I don't have a life right now other than school. I feel myself getting more tired and I have less energy than I'm used to having. School is over in May and then hopefully the studying is over in June/July when I take and PASS (fingers crossed) the nursing boards. Until then I really hope that my best friends and family can understand that I am not avoiding anyone, I just really need to get through school and keep myself as healthy as possibly at the same time.