Babe it will take a bit of preperation,,

you may need to get yourself loosened up some prior,

my own man is kinda on the big side and i have had to wear a butt plug for the anal side myself prior to help get me used to be being filled and be able to take him without killing me back there,

i actually found sizeing up gradually over a week or two works wonders in the long term smaller to larger until you can accomadate him without clenching,, also when he goes in, you push out and back like your going potty, it eases entry

lots of lube and foreplay for you sisa have a few orgasms first before he gets any where near the penetration part. If you cant phycologically divorce your body memorey from the pain every time he goes to mount you will tense up, it will take lots of time to change that now that its established