Throughout my work as a Midwife and a Doula I have encouraged and supported people to have a positive mindset surrounding their birth. I've created these free D for Doula's Birth Mantras, scroll across through them (below).
Be mindful to read them when you are in a state of relaxation and contentment. Print out and post around your home to remind you. Have these mantras visible when you practice your breathing in labour exercises too, see video here. 
I've added a blank birth mantra below, for you to download, print and write your own, If these don't resonate with you,