What’s the hardest part about predicting your monthly spending?

I’m working on Intelens, a tool that helps people track expenses and predict where their money is going before the month ends.

While building it, I realized most finance apps show past transactions but don't really help you anticipate future spending.

I'm curious — what do you personally find hardest when trying to predict or control your monthly spending?

Is it subscriptions, irregular expenses, budgeting discipline, or something else?

Would love to hear how others deal with this problem.