Why is it so hard for me to keep friends? help plz :)?

All my life I have been reserved or shy. I'm 18 now and im in college. I have 2 bestfriends that understand my personality and are nice. I hate talking and sometimes only talk to them once every two weeks. My bestfriends don't care, they still been my friends since we were in 1st grade but, its harder to keep friends now im going 2 college. How do I keep a friend in college or what's the best way to make friends? thanks :)