I've been on a low carb diet for 5 years now. The key I found to "sucker for sweets" desire is you take one day a week off. I find sugar is like salt now... if you give it up for a while and go back it's really really sweet and after a day of enjoying sugar I'm ready to stop eating it again for 6 days. If you haven't tried the one day a week off I recommend it next time you do your 3 months.

I did keto for 4 months at the beginning of this year, and I did lose 40 pounds, and generally felt great. But then I started getting more active and picked up tennis. I found that after 30 minutes, I was completely out of energy. It wasn't that I was tired, it was that I literally felt like I was out of fuel. Taking a swig of Gatorade immediately picked up my game, and so I had to introduce carbs back into my diet just to stay active.

That's quite the contrary to my experience. I combined the diet with a daily 5km morning run, and I had no problems with it, not only was I not exhausted, I was powered up. But maybe that's just placebo :)

I expect you were missing electrolytes, not carbs. That's what the Gatorade helped with.