so, for more than a year I had anal pain just once. It was my fault. I inserted the big ball (on the "Groove") too fast, without good stimulation and relaxation. It took one week to cure.
As I wrote, I started with
http://gadgets.boingboing.net/filesroot ... 00x200.jpg it relaxed my sphincter and I realized that this is very interesting idea how to cure anal fissure.
Than I bought
http://www.amazon.com/Platinum-Pure-Silicone-Groove-Blue/dp/B00A8VHATK It has very narrow end and really good for the start.
Now I'm using
http://www.amazon.com/Exotic-SILICONE-LOVE-RIDER-KISS/dp/B00E8R2HJI for fun and to keep my sphincter "in shape"
Use lube and give it some time, do not use big diameters when your sphincter is not relaxed enough.
To avoid constipations I eat yoghurt.
Good luck!
oh, and don't forget always to wash your butt thoroughly! remains of feces cause pain!