I have the benefit of having two pre-op diets to reflect upon.
The first time around I toughed it out on sheer determination. I thought that I was doing well, but about halfway through I started dreaming of mashed potatoes and spaghetti!
The 2nd time was more than 10 years later and really, for the most part, I was back to square one. The difference was that my new surgeon didn't have me on liquids only. I had a protein shake for breakfast and lunch with a "sensible" meal in the evening. Enduring this was MUCH easier, but I had a bad case of carb withdrawal after about 3 days. I had the worst headache that was constant for about 3 or 4 days and NOTHING helped it. I had brain fog, fatigue, irritability. I'll take the dreaming of mashed potatoes and spaghetti any day after that!
Hang in there! It's a temporary situation and this will pass. For me, I just kept my eye on the prize. That was inspiration enough to help me stay on track and push forward.