Pre-Op diet isn't easy. I have admitted on here several times I cheated and slipped up during the 2 weeks I was on it. Cheated like going through McDonalds and sitting in the Target parking lot mowing down a Big Mac, Fires, Etc thinking this was the end of the world. It is a short phase and will pass. Stay strong.
You can buy ziploc bags in the shape of a freezie and add 0 calorie drink flavours (cool aid etc) and I hate the shakes, so I lived off optifast shake cookies (add 1 egg white and 1 cup splenda) and bake for 10 min, at 350) it just adds 10 calories and adds protein. You can have as many freezies as you want in the evening. I am post surgery and still have them as a treat and helps to get my water in.