Your thoughts become action.
Your actions become your habit.
Your habits become your character.
So, mind your thoughts.
Thought → Action → Habit → Character.
A single thought is almost harmless. But once we build on it with more and more thoughts, that's when things start to happen - for better or worse. And it becomes harder to break the thought pattern.
So, watch that first thought. Consciously decide whether to keep at it, or to let go.
A single thought is like one brick. Not much impact. Accumulated thoughts are like one brick on top of another, and before we know it we got a wall in front of us. Now, the impact is more with a wall. It is no longer insignificant.
So, watch that first thought, or even the second, third.
You have a choice.
Decide if you want it to become your habit/character, or leave it at / as "It's just a thought."