Do people never move on from their first love ?

When it comes to love and relationship I’ve never experience it . I’ve heard about this first love theory ..even my mom still remembers her first love and whenever I talk to any guy they also talk about their ex how they still remember their first love and that’s make me overthinking . I’ve never been in love I was never someone’s first love and I don’t think I will ever be someone’s first love cause I’m adult now and everyone usually had their first love in teenage ..my biggest insecurity about relationship is if I get married to someone or got into relationship what if my partner still think about his first love ? What if he tells me he loves me but still has his ex on his mind ?So I want to ask ..is first love that important ..that unforgettable that you remember it your entire life