I agree that trust in your partner is key, but so is the woman's willingness to relax and be stretched over time, which is not the same thing. I always needed enemas before trying vaginal fisting. I have given birth, but always tightened up again afterwards. I did my kegel exercises, which helped a lot, of course. So it was hard to take a man's fist, it took months of work. My husband always insisted on a series of enemas first, for reasons stated elsewhere in this forum, making room inside for the whole hand.
The best orgasm I ever had from fisting, and I never failed to have a world-tilting orgasm or three each time, was when I had a douche nozzle in my rectum, taking a slow hot enema, lying on my back on a towel in bed. My husband slowly stretched me out over a long time, and finally got his knuckles in, made a fist and fucked me REALLY slowly. He let the enema keep flowing, I had so many sensations inside my pelvis! I cannot remember if I rubbed my clit or not, he was busy using one hand inside me, the other slowly helping to open the labia and check to be sure he wasn't going too quickly.
It took forever for me to come, the pleasure was so slow and SO intense but I never quite came until all of a sudden the orgasm slammed into me. I'm pretty sure I screamed as though I were being murdered, but there is no way I could have kept silent. It lasted a LONG time, I can still remember how powerful it felt.
We didn't have to use any lube other than his saliva and my own juices, which were SO thick and so slick and so copious his hand felt just right, even when the most difficult part, the knuckles, went in. Sometimes during our fisting sessions I had a little tear but it healed in just a day or so.
I haven't done that in so long. Aargh! I want it, right NOW!