It's probably not you, it's just badly structured legacy code, and you're trying to understand it all like you wrote it. Been there, it sucks. Find someone who

It's probably not you, it's just badly structured legacy code, and you're trying to understand it all like you wrote it. Been there, it sucks. Find someone who worked on it and have them explain it to you like you're a 5 year old. If they can't explain it in simple terms, you'll confirm that it's not the piece of cake you thought it was. I don't dismiss that you might just need some sleep and are overworked, but since you're asking these type of questions, I'd say you're probably fine.