To work out our salvation with fear and trembling. Meanwhile never internalizing the commandments of the Lord. The scriptures say that your heart is assured by keeping the commandments. I don't quench that by taking some Bible verses out of context. Many of those that doubt ain't saved and at a minimum are not serving the Lord in spirit and in truth.
Fear is like pain.
A man may have pain in the abdomen, so he goes in to the doctor. It's okay for a man to be sorry or scared after a godly sort. There are people who for some medical reason cannot feel pain. These people do not live long 'cause they can't tell when something is wrong with them. Pain is unpleasant, but it is good because it indicates there is a problem.