My wife once posted on another forum to answer this question. I'll paraphrase.
She said that I was successful in intrucing her to enemas because I first sought to fully satisfy all of her other needs, both in and out of the bedroom.
In terms of the bedroom, what always worked for us was to have her already over the top, whether thru oral, intercourse, whatever. I'll quote my wife in saying that "once he had me cumming, he could have hit me over the head with his shoe and I would have loved it".
So, please your partner first, have them in the throws of passion, and the potential for it to feel good to them is greatly increased. And that is the key....making it feel good for them.
In terms of the discomfort on the toilet, etc, that can be addressed. Don't send them off to the bathroom to fend for themselves. Women can orgasm while expelling, and trust me, orgasms make anything better ;-) You don't want to tell them what's going to be happening, just guide them during the experience, and keep her feeling nothing but pleasure.
I'll admit that I introduced my wife to enemas very early in our relationship, so I don't know how all of that would work with someone you've already been with long-term. Good luck!