Why can't i just tell him!

Well let me tell you a story
About a girl and a boy.
He told me i was his Bestfriend
and Nothing and Noone would ever change that,
He told me that he would never let anyone come between us,
He told me he wouldn't know what he would do if he lost me,
He told me he wouldnt forget about me,
He told me that he would always be there for me,
but people tell lies and hes full of them,
because as soon as she came along,
i was nothing to him anymore. I guess it was a good because that is what i want but i don't want that anymore! i have to get him to let me back into his life even if its only for a little while.I have to tell him the truth that is i have cancer!