Always done great on greenfield projects, but suffer on anything else?

Most of my tech career, I have had the chance to work on greenfield projects. Now, for the last few months, I have been on a team where we're maintaining some existing software.

To keep it short, I hate it.

I find the codebase is unnecessarily complicated, and I am having a hard time adjusting to the team's way of working.

What do I do? I brought this up with my manager and he rightly said "sometimes we just have to do the work" but short of quitting I feel like this sucks.