I feel like it happens a lot in modern relationships. People become so interdependent and build themselves and their identities propped against each-other (to the point of neglecting families and friends even), so much that when it chatters it's a life changing experience. The interdependence could even be a cause of the breakup itself. I also feel like a lot of dysfunctional couples are staying together not to break the illusion (some kind of weir sunk cost fallacy).
People need space and solitude to find themselves, only then they can start building healthy and stable relationships, ~50% of married couple divorce (and much more unmarried breakup), statistically your couple won't last, live like every day is the last day of your relationship and make the best out of it (it also helps keeping the gears of love running).
Treat love as a drug (or food), when used in moderation and in a safe manner it can be soothing and positive, when your life starts to exclusively revolve around it it quickly can spiral out of control. Maintain a good routine; friends, hobbies, sport, job, &c.
Read books about the psychology of love, couples, relationships, breakups, it helps a lot when you figure out that most people are going through the same thing and are getting past it with time. You can also check out stoic philosophers and how to live for and by yourself, you don't have to take it to the letter but it certainly gives you another point of view.