Overwhelmed by a shitty codebase? Use the boy scout rule and leave the code you're editing a little better than you found it.

Worked wonders for me when I realized I could spend literal months refactoring and desperately needed a systematic approach.

Little by little that rotting house of cards will turn into something okay. It's a nice feeling looking back after a couple of months and see what you've done to make things better.

Also, make sure to remember the cost of wrestling with hurried legacy solutions in your estimates as well. Just adjust the level of bluntness depending on your work environment: admitting that things can/need to be improved can be unpleasant for some to hear even though it's true.