You have to be 100% un-precious about your work, happy to scrap stuff you have worked on for several hours , undo-changes, and at times basically start from scratch.
While as an artist you are working for yourself, as an illustrator you are working for someone - and not vaguely so either, the results have to work for your publisher. And the more important the publisher usually the more stringent their demands will be.
This deserves mentioning because so many artists are extremely precious about their finished work. They come from a 'personal work' background, and balk at being asked to change things in their first forays into publishing.
The sentimental idea of an artist treating their artworks as their 'babies' is also very pervasive in our culture, so that the illustrators who are suitably nihilistic about their work and change things up to fit briefs constantly actually keep it under very tight wraps, for fear of seeming mercenary.
NEWSFLASH: The best illustrators are also mercenary. You need to be ruthless about making things work for a client, rather than for you. If that sounds unromantic then this job will never fulfill you.