One of the greatest difficulties that I have with fasting is thinking that I've learned what I need to learn and it's time to stop. It would be convenient if fasting came with directions like paint by numbers art, but it's more like microwave popcorn. You put it in and have to stay there and wait. Sometimes nothing pops at all and sometimes you burn the bag hoping for the last kernel to burst, but more often than not, there's delightful aroma and feast in store.
I tend to want to stop a fast too soon, not reaping the fullness of reward that I could; not knowing that there actually could be more in store for me if I would persist through those "good enough" feelings.
I actually had this very thing happen in a recent fast. I thought long and hard about exactly what to give up and then finally choose something on the spur of the moment to give up for an intended month. It was terribly tough the first couple of days but after that part was done, I thought "Great! Awesome stuff I've learned! I'm going to stop now." But then I continued and learned even more, something even better than I learned before and I thought, "Wow, I would not have made that connection if I had stopped!" It was good stuff. The things I've learned have even caused me to wonder if I should remove this thing from my life completely. Fasting is not cut and dry it's a take it as it is kind of thing, you have to be in communication with God about His desires for your life and be willing to push on beyond your comfort, with the hope that something wonderful is in store for you. We can't be concerned about ruining things by fasting or what we will miss out on by spending time with God in this way. We need to focus on that wonderful aroma and great feast that is waiting for us.