You aren't alone. Other people have had to deal with work/school things keeping them apart for a time. It is what it is, and once the work/school is completed they return to where they can be with their loved ones. Think of deployed military for instance.
You will get through this. Nursing school is not forever. Daily calls with both and weekly visits from B -- they are trying to do what they can.
Are you sure it is not "I feel lonely at nursing school" and it could be ameliorated by making some nursing school friends? Other nursing students would TOTALLY get the load you are under right now with school.
Or could it be "I feel jealous
" of some flavor? Or more like ENVY? You don't want to take anything away from them, but wish that you had what they had -- the time to spend with each other.
Hang in there.