Writing new stuff from scratch is fun, I have zero problem being motivated to code things that are hard or challenging. And I can keep focusing for a long time.
However, if my task is to shoehorn some code that I know to be crap into my project for x or y reason, oh boy, then I can drag my feet for a looooooooong time.
That's one of the reasons why I don't like working on UIs. The code tends to be messy. I often have to test in environments like IE for Windows. And I don't actually care about how things work as long as they work.
That happens sometimes. It is then that I remind myself how much a I make a minute plow through it.
So much code no matter where you go will be something like this. You'll have to make something work even though all the code surrounding your feature is a dumpster fire that is so hard to predict, that you'll pull your hair our worrying about all the ways your feature will break.
When I get to build a new system from scratch? It's a breeze! I get thousands of times the amount of work done in the same amount of time.