I am 40 now and a better programmer/developer than I ever was before. I still encounter similar situations as you but I think you make a mistake by thinking your younger self would have handled this (easily).

The difference is that our younger selves were maybe more optimistic and naive and probably would have just produced a new mess for the next one to scratch his/her head about.

Abstraction is hard and we usually have to much of it in our code bases. You have to break it down and isolate smaller tasks to get started with. Start with some endpoint of the API and try to understand what it exactly does. Usually there also should be an API spec where the ins and outs are defined. Maybe it is easier for you to write this code by adhering to the spec and/or fulfilling existing tests than reading the old convoluted code?!

Probably you know all this and maybe you just hate your task. If this is the case.. sometimes a change is what you need.