Your relationship started when she was 16yrs old and you were 17. She was a mere child and may have been in a relationship with you because that's' what girls do. Actually, you were both too young to be in a committed relationship. Now she is stretching her wings and understandably you are freaking out. You breaking it off now is the smart thing to do. Do it amicably and tell her you hope she finds happiness and you will do the same.