As soon as I start making positive progress towards a goal, a habit, a skill, making money or my health, there soon comes the feeling where I think I can now slack off. It's kinda counter-intuitive ... I mean, why ruin a good thing? But when you think of us as a primal species, the logic makes sense.
For starters, our mind and body wants to always be comfortable. Any threat to one's comfort is marked with a barrage of inner questions, making you second guess a new opportunity, or hold back from going hard in the gym.
If I work hard on something, and inevitably improve, then my body is now set at a new level of performance. That's exactly the time where the lazy mind likes to say "hey, you've done well kiddo, now that you're above your old self, there's plenty of room to just take it slow..."
So it's like, the body is trying to say, "you're above your average, so it's time to coast and take it easy now that you're better."
Aaaand then if you do that, you will in fact, slip and go back to where you started. This happens to me all the time, especially with exercise and PB's. One month I'm crushing, the next I'm back down where I was two months before!
Why can't we just keep persevering ... keep on going, when the going is good?