What is a synonym for “I should have, but I didn’t”

Question

E.g (2):

I should have stopped him before it was too late, but I didn’t

I should have provided a source, but i didn’t.

I am looking for a single word for the entire sentence; a synonym. I’m not looking for the word “regret”, but something sort of alike.

Answer 1

failed

to not do something that you should do

fail

So your sentences could be:

I failed to provide a source.

I failed to stop him. or

I failed him.

Bonus answer: neglected