I'm coming from a slightly different perspective as I've had breast cancer, I was 46 when diagnosed so had never had one before and was dreading it but I didn't find it painful just a bit squashy!
It actually didn't pick up my cancer, an ultrasound did so I'm not 100% convinced about them as a sole detection source but of course I have them regularly at my check ups and even post surgery and treatment I don't find them painful.
Don't fear what it may find, you really don't want an undetected cancer running rampage inside of you, my mum refused to have a mammogram and sadly she died from secondary breast cancer at 60.
Tell them you are anxious and they will be gentle with you, all the best! Xx