I think fun is a misleading term for GMs.
Running games isn't meant to be "fun" or "entertaining". You're not doing it to get the kind of adventure that you want to see. It should definitely be enjoyable and rewarding, otherwise there'd be little point in doing it. But there are many kinds of work that are very enjoyable to certain kinds of people, even though they are work and not play, and you couldn't call them fun. And plenty of people do them for free, investing their own time and effort.
I think that's a much more productive approach to being a great GM than trying to make the game "fun" for yourself.