Where a mortgage payment actually goes

The payment is constant. What it is made of is not.

Each period, interest is charged on whatever is still owed. The payment covers that first, and only what is left over touches the balance. Early on the balance is large, so the interest slice is large and the repayment slice is thin. Late on the balance is small, so almost the whole payment is repayment.

That is the entire mechanism of amortisation. Everything that feels unfair about the early years of a long loan falls out of it.

Two useful readings of that chart.

The early years feel like nothing is happening because nothing much is. The balance is what interest is charged on, and the balance has barely moved, so the interest bill has barely moved either.

And an extra payment made in month six removes interest from every one of the 294 months after it. The same payment made in month 290 removes interest from ten. The value of paying early is not a moral point about discipline; it is the number of future charges being deleted.