Why do I underestimate my ability as a writer?

 So I could please the client

When I was on Upwork, I used to underestimate my ability to write for two reasons. One was to give the client more than they were expecting, and gain praise for my work, and more orders - selfish I know - the other reason was by knowing I could do more than I said, I could fullfil the order and not go beyond what I could do.

Over the years, this tendency not to give myself praise has annoyed several people I knew, my view is self-praise is no praise. I would rather copy and paste praise from a client to boost my reputation, than praise myself.