Know this, not every person who comes into your life will be a lifetime friend. There are are people who come into your life for a season, during which you both will hopefully share, learn and grow.
When the time comes for them to disappear, they will, despite your emotional pain in letting them go. Your effort to hold onto them will be futile. For as seasons come and go, the same is true of people. Your relationship will end at the appointed time.