TL;DR: Don't rely on the relationship for happiness, have you're own hobbies, interests. It is easy to loose them without realization.

Say you just broke up or got broken up with, most of you will get feelings of sadness. Your mind might start racing and making connections that don't exist: she broke up for a reason, that must mean I'm crappy/shit person/bad. Or that you miss her: did I make a mistake, I'll never fine anyone like her. So in summary, most people will feel pretty shitty after a relationship ends.

But why? Sure you miss her but you'll probably find someone better as long as you're out bring a social butterfly. However, you'll tend to connect happiness because you have a girlfriend and socially that must mean you're okay. In reality you're not being the person you want to be and you're using the relationship (just as well as videogames, alcohol, weed, etc.) to convince yourself that you're happy because you're happy doing them. So when they leave you get sad because that person affirming that you're okay and happy is gone.

PSA: Of course if you loved someone and miss them after a breakup it's harder because of the strong bond you had (that takes time and nothing else), but this is a good realization to let go and move on with your life. This obviously is not applicable to abuse and like it.

However, that also means you're not a bad person. You are still okay. Your life isn't over. It is always possible to move on and become who you want to become.

Even now just reflect on your life and write down all your hobbies that give you genuine happiness. If there aren't too many, write some that you think you'd like and try it! Start small for God's sake, just try something out. There was a great post here that listed a bunch. It's your life, which means you can't be dependent on someone for your happiness. You're the only one who can create your own happiness; having a LTR normally should amplify things.

TL;DR: Don't rely on the relationship for happiness, have you're own hobbies, interests, etc. It is easy to loose them without realization too.

If this is not applicable to you don't fucking be a brat and tell me how my perspective is wrong, I don't care about your two cents.