Hi :) some of the tough things are creative burnout especially in an in house work environment where a designer works on the same kind of projects most of the time. It can also be tough to be always professional l, and not take it personally when what you think is your best work ever doesn’t get picked or used for business (money) reasons or someone in charge does not get it. Or you are asked to change it and you totally don’t agree with what is being asked of you. These are times when budgets and office or company politics come first before design excellence. These can be quite painful moments but the longer you work, the better you will be able to handle them. You need to be flexible and learn to pick your battles well. If your work is excellent and highly valued that will be a huge plus. Working at a place where you are excited to be is another great way to balance things out. There will be plenty of super rewarding things happening as well and hopefully they will way outweigh the challenges.
To me, the most challenging part is to be creative 24/7. However, nowadays I love it. As in everything else: the more we practicing, the better we become.