In order for "anal" sex to be enjoyable for both partneres, it can't be forced but has to be approached either cautiously or playfully depending on your relationship. And you can also lead up to it during foreplay and see how your girl responds. Possibly the initial feeling with get her in the mood. And remember, in addition to the nerve endings giving pleasure, you can attempt to stimulate her g-spot from the front at the same time. Also be cautious of the problems with gettting hemorrhoids from excessive activity down there. Maybe some aftertime anus massage attention -try olive oil and tee tree oil which actually have curative properties?