Does that mean I have to pay 15% tax on all my long term capital gains?
No. Taxpayers in low income tax brackets (10% and 15% brackets) have special lower maximums. If you had other regular losses, sometimes there may be no tax at all.
The capital gains tax rules also apply to net capital gains or losses. A taxpayer’s capital gains and losses for any taxable year must first be netted, with short-term gains and losses netted first against each other and long-term gains and losses first netted against each other. Then the net short-term gain or loss and the net long-term gain or loss are netted against each other to determine the net capital gain or loss for the year.
There are many special rules dealing with capital gains and losses and with the holding period for assets. Your should check with your tax or financial advisor to see if any of these apply to your situation.