I’ve come to the same rough timing as @lmb independently (currently using 45/15 – but because I’m doing “work cycles” the 15 includes deciding what to do the next cycle, which takes a few minutes, so really it’s identical to 50/10!).
I do let myself work through the 15m boundary sometimes, but it sounds like you need to take it to the opposite extreme of sticking to it rigidly.
This has a tiny bit more structure and may help: "Work cycles" - #4 by halfplane