must resign during maternity leave and reapply-help

i have worked in this hosp. on the same unit for 5 yrs. i am expecting my 3rd baby the end of july, but have been taken out of wrk recently due to complications. i am per diem so i am only entitled to one schedule period off ( 1 month ). i have spoken with my manager and HR and filed the needed paperwork and what it comes down to is that i have to be back mid August-the baby will barely a month old, or resign then reapply for my position ( which i'm not even sure they will repost since its per diem and the fact that my old manager created it for me upon return from my last leave). so i would like advice on what the resignation letter should say. i would just like to get ahead start before the baby comes and i'm overwhelmed and over emotional about having to resign. thank you!