Why are my early mortgage payments almost all interest?

It's how amortization works: interest is charged on your remaining balance, which is largest at the start, so early payments are mostly interest with only a sliver going to principal. As the balance shrinks, the interest portion falls and the principal portion grows, until late in the loan nearly all of each payment reduces principal. On a 30-year loan, you often don't cross the halfway point on principal until roughly two-thirds of the way through. This is why extra principal payments early have outsized impact – every dollar paid now removes interest on that dollar for decades. Ask your servicer for an amortization schedule so you can see exactly how each payment splits, and how extra payments reshape it.