Fisting's not for everyone. Especially if your vaginal canal is small.
If you wish to enjoy this with your partner. Make sure his fist is saturated with lube as well as your pussy lips. The wetter the better. Less friction.
Have him insert 4 fingers (straight-not bent). Then slowly have him insert his thumb. And as he's slowly inserting all five fingers have him gently close his fingers into a fist position. IF you feel pain. Try to get yourself into a different position (while inside). Sometimes laying on your back can cause the muscles in your vaginal canal to be too tight. So try it in doggie position and have him insert his hand from behind. Your vaginal muscles are a bit more relaxed in that position. And then slowly have him move his fist in & out. building up to a faster rhythm that your body can handle. All the while doing your kegel exercises and putting a grip on his fist.
Afterward, have him insert some ice cubes and or a frozen Popsicle. To cool you down inside and to bring the tightness right back.
Fisting is a feeling like no other. The bone structure in the hand lends to a totally different feeling unlike anything else. It heightens ones climaxes and massages the vaginal canal in ways a cock doesn't.
It's just another sex toy of the human kind!!