It's just a memory like other incidents. Hence if a person can forget his/her other incidents easily then he/she can forget his/her first love too. All depends on memorizing or how impacting it was on one's life.
Very hard to forget. But sometimes I do not want to run into her either. It's a small town I live in. And her friends are some of my friends girlfriends or wifes. If invited somewhere to meet with friends and I know she's gonna be there too. I won't go.