What if my project fails?
What if I'm asking the wrong questions?
What if I'm analysing my data the wrong way?
What if someone disagrees with my ideas?
What if my hypotheses have already been disproved elsewhere?
What if I break my leg?
What if I fall pregnant unexpectedly?
What if I get robbed?
...Then I get out of bed the next morning, with my family and friends to support me (because they won't love me any less), and do my best to carry on regardless.
I know that being thoughtful is an important part of being a scientist. Also in being a happy human.
But I think a lot of people can agree that it's far easier to sit on either side of the scale of "too thoughtful" and "not thoughtful enough" than it is to sit in the middle.