Prevent myself from crying?

ive been with my bf for 4 years and i jus started recently crying when he leaves. he comes over every friday. if he comes over friday and leaves friday i dont cry but if he spends the night and leaves the next day i start cryin when he leaves. i dont plan on crying it jus happens. is there anything i can do to prevent myself from cryin over him. he holds me for a while and i know i cant stand there forever.i dont want to cry it jus happens