Spending time trying to get to know someone only for them to leave... is it worth it?

I have nothing against them, I believe in letting people do what's best for them and letting them choose what they want even if that's doesn't include me but what am I supposed to do? I'm left with memories that weight me down, promises they couldn't keep and mixed feelings of love and betrayal for them. And no matter how much time has passed, it's still "I love you and I hope you're having a great life but I also hope to never see you again because it still pains me how much you've hurt me." and I also can't bear to see your happiness knowing you left me, cowardly and unapologetically, in my deepest moments to go and live your life and now, years later, I'm still fighting for it while you get to enjoy..
But the main question is: is it worth it? Spending time, money and energy to build a relationship, putting effort into little surprises and gifts for big days and celebrations only for it to become a bitter memory, watching you two drift apart and dreading the inevitable while also pretending you're fine and you'll be fine...only to realize it's never enough. You're never going to feel happy enough or safe enough or over it but you also can't go through life without making any relationships, can you?