Or is it dangerous, toxic?
i think it depends on how you react/live after you break up with your lover. If you can live a normal life without them, meet other people, care for yourself, see yourself as a one self and not a “two self” anymore, then it’s okay and your former relationship is just a life moment, a life experience.
But if you can’t live a normal life without comparing everything to this former relationship, then it’s toxic because it means this relationship shaped your mind, hence why it’s good to experience different things early on and to have different lovers while still younger, because later you have plenty of time to settle down and it’s not during your studies that you’ll grow a real proper family