My best friend is also my roommate and I'm going to tell her that I no longer want to share the food that I purchase with her.
Is there a reason you want to stop sharing your food?
I feel like she takes more than her share.
If you think your friend has been taking advantage of you by eating more than her fair share of food, it's okay and understandable to want her to pay for her own food. It might be awkward asking to split expenses at first but you should explain that it is more fair to do things that way.
It's easier said than done, but my mind is made up.
Roommate situations can be prickly to deal with, but it's reasonable to split expenses evenly. Having a conversation about paying for your own food will prevent future issues and resentment.