Oh I am so very sorry you have this. C Diff is difficult. I know your doctor is trying to heal you and I don't know the ingredients of the Culterelle brand.
I do know that probiotics are super important for this problem. You don't have any good bacteria in your gastrointestinal tract.
I think she is being cautious in only having you take an over the counter product. I do not think it will hurt you.
If it were me, I would add in some fermented foods during the day. Not whole bowls full, but maybe just a teaspoon here and there of kimchi, kombucha, sauerkraut or a pickle.
I don't know how bad your c. diff is or how life threatening it is. I have a long time friend that is really bad off. She is now contemplating fecal transplants.
Here is what my buddy Dr. Axe says. I rely on him a lot...... /draxe.com/fecal-transplant/