Obviously, you need to do your budgets on the 40 hour week

If any overtime shows up, that’s a BONUS. But you can’t count on it anymore.
What’s the LOWEST amount you get for your pay? Use that amount for budgeting. Again, anything above that is a bonus to be thrown at whatever step you’re on.
Sounds like your situation changed, and you just didn’t adapt quickly enough. If your bills were $2000 per month and you were making $2200 per month, then you’d have some leeway. If your income has dropped to $1900, then you either need to drop your expenses to $1900, or increase your income by $100 somehow.
Again, it’s all mathematics – they don’t lie.