How do you know how much of the money in your bank account is free to spend?

You don't need the notepad, and you don't need to log every purchase to answer this. The trick is to figure out the number once per paycheck instead of every day.

When you get paid: 1) list everything that has to be paid before your next paycheck (rent, car, insurance, subscriptions, minimum payments), 2) add what you want to save, 3) subtract both from your paycheck. What's left is your "free to spend" for the pay period. Divide it by the number of weeks and that's your weekly number.

Then the only thing you check day to day is that one number, not the bank balance. Many people keep bills in a separate account or a note in their phone so the bills money is "not touchable." It also helps to line bill due dates up a few days after payday.

Tracking every purchase is useful for the first month or two to learn where the money goes, but after that it's optional. The plan does the work.