Confidence comes from doing something and getting good results. When you get bad results or mixed results your confidence will waver. Doubt will set in.

You conquer doubt with practice. Practice of doing it the right way. It takes a unique amount for each person.

but I wanted a human approbation

Here, I would trust any calculator over 10 bobbym's any day of the week and twice on Sunday. Look at what it did:

Now anyone could do it by just following the rule that parentheses and nested ones go first! Wunderbar!