All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Sister, you are required to immediately observe the days that you have missed, as soon as possible. One never knows when his life might end. Ask Allaah to forgive your delay of making up for these days in their due time, which is before the coming of the following Ramadan.
You also have to provide food in charity for each day that you missed. You have to feed poor people in the United States. If you could not locate poor people over there, then send the food to any Muslim country either by yourself or through a charitable organization.
Allaah Knows best.