If I am uncertain of something and later I made myself certain again, is there a single word to express that?
“I was uncertain whether the result was correct. But I
re-certainedmyself by double checking my calculations.”
You could use the word reassure, about which the Oxford Dictionaries says
Say or do something to remove the doubts or fears of (someone)
She’s planning to be a volunteer teacher; it reassures me to know she’s got such great aspirations.
So you can say
I reassured myself by double checking my calculations.