John Newton, Letters of John Newton (Banner of Truth: 1869/2007), page 380:

I have no doubt you think others better than yourself: thus far you are conformed to the Scripture rule (Philippians 2:3 [“in humility count others more significant than yourselves”]). You should not therefore be displeased with yourself on that account. I shall not contradict you. But some of those you deem so were planted in the Lord’s garden many years before you. Why then should you complain that you are not so tall, nor your branches so wide, nor your root so deep, in two years’ growth, as others who have been growing twenty or thirty years? Should a little sapling, just springing up from an acorn ask, “Why am not I as large as the stoutest oak in the wood?” You would know how to answer it. …

Do not let Satan impose a false humility upon you. Depend upon it, there is more of self and self-righteousness in these complaints than we are usually aware of. It is better to be thankful for what you have received than impatient because you have no more. If you can make yourself better, do it by all means; but if you cannot, wait simply the Lord’s time, at the Lord’s feet. If your heart is upright, you have only to attend to the means and precepts of grace [private prayer and Scripture study].

The Lord must do the rest, and He will, otherwise it can never be done. Try to be thankful; it is both a utile [useful] and a dulce [pleasant]. You cannot be too humble, or think too little of yourself; but these views need not break your peace. You are to be strong, and to rejoice, not in yourself, but in Jesus Christ the Lord, and in the power of his might. One view of the brazen serpent will do you more good than pouring over your own wounds for a month [Numbers 21:4–9, John 3:14–15].