When you try to lose weight too fast, it backfires because your body goes into a hypometabolic state. It thinks it's starving and becomes very efficient at saving calories. Your energy drops. You have to keep your energy level high.

I lost over 60 lbs several years ago and I've kept 40 lbs of it off. My personal experience has been that it's good to alternate days that you have a big calorie deficit with days where you get enough calories or even a few extra calories. You have to shake things up a bit. I also found that my weight loss would show up the morning after a day that I ate a lot. For example, I ran about 5 days a week and watched my calorie intake closely. Friday (the 5th day of being "good") the scale wouldn't have budged. Saturday, I'd skip working out and go out drinking and consume a lot of extra calories. Sunday morning, the scale would show I'd dropped 2 lbs. It worked that way pretty consistently.