I was on 3 shakes a day and sf Jello or popsicles and I did not cheat. Use the time wisely to keep busy, clean out your closets, prepare for surgery, up your Water intake, take walks. Day 3 & 4 were hard. If you used to be a pop or coffee drinker, many of your issues may be caffeine withdrawal.
If you cheat you are only cheating yourself and since you are going into such a major life-altering surgery try finishing this time out giving 100% dedication.
It's not easy, but nobody said it would be. Have faith in yourself that you can do it and go into this the best you can. If you cheat, dust it off and then realize you need to put in 100% effort.
You can do it.