Capital gains is essentially broken up into short term or long term. Long term is holding an investment one year and one day. If you hold it less than that and sell it at a profit, your profit is taxed at your normal tax rate. If you sell it after one year, you are (currently) taxed at 15%.
Just look at your trade dates on when you invested. Those trades should be held 1 year and 1 day before you sell if you want to be taxed at the lower rate.
If you want to argue why it's such a big difference, or seems unfair, know that you are being taxed on money that was already taxed. Many people think it's unfair that "rich" people pay "lower" tax rates due to long term capitals gains being only 15%.
My personal opinion is that it's pretty awful that you are taxed when you make the money (paycheck), taxed when you spend it, and taxed when you invest it.