Welcome Informed Criticism of Your Work

When you come to 'know' something, there is a temptation to stop thinking about it. You put it in a box as 'known' and are happy to argue with anyone who disagrees.
This is both a good and a bad thing.
It is good to hold firm to your beliefs, to be true to yourself, to stand your ground over knowledge or a belief that is fundamental to your character and values.
That said, holding stubbornly to your point of view, allowing no space for doubt, can be limiting. Oliver Cromwell is reported to have said to Charles I, on the eve of the king's execution, "I beseech you, in the bowels of Christ, consider is possible that you may be wrong." Or words to that effect. Charles I was clearly in no mood to change his mind, and the outcome was not pleasant for him.

Holding true to what you 'know' in terms of copywriting is clearly small potatoes when compared to the decision Charles I faced.
However, we are still faced with the same questions. Should we always hold fast to what we know about our craft? Or should we allow for the possibility that there are better ways to write, different approaches to take, alternative words to consider?
No and yes. No, we shouldn't back down every time a client or colleague questions our work. Yes, we should consider the possibility that what we knew from last year may not be the best choice this year.
So at least, when you have written some copy, seek out a contrarian view. Find someone who's opinion you respect. Ask them to read your copy carefully and criticize it in any way they want. And then consider their views carefully.
Discard criticism that you truly feel is not on target. But also, leave yourself open to the opportunity to grow as a writer, expand your knowledge and skills, and find new, better ways to extend your craft.