How can i convince my girlfriend to take part in this fetish?

I have a fetish where i want my girlfriend to hold in her poop for at least a week (2 weeks would be even better).

She could take some kind of pill as well that would constipate her and not allow her to have a bowel movement for a week, or a bit more.

She usually is very regular, and poops at least once everyday, sometimes twice a day, so to think that instead of all her feces being dumped in the toilet, instead they would pile up inside of her abdomen, is really something i want to witness.

I just think it would really be a turn on for me to know that her feces are building up inside of her intestines, day after day, while she has a look of discomfort on her face.

What i would especially look forward to would be for her to hear me take a crap everyday, while she is holding hers in, which would surely make her even more desperate to go.

Furthermore, as we go to bed, next to each other, it would be especially sexy at that time, for me to know that she has a week worth of feces inside of her system that are slowly rotting and festering away inside of her system, as we are cuddling up under the covers in a very intimate manner!!

When i tell her that i would love her to do this for me, she gets freaked out. How can i convince her to do this for me???

I'm just really turned on by the thought of her poop ROTTING away inside of her for a week, as the look of pain becomes greater and greater each day.

How can i convince her to do this for me? Any ideas?

Thanks everyone!