These relationships occurred during your teens and early 20s. You were and still are growing as person and establishing your identity and what you want out of life. Our brains don’t even develop fully until our mid-20s, so it makes that you might grow out of your feelings for someone you chose when you were 20. I don’t think this means there’s anything wrong with you, but talking things out with a therapist is always a good option and can only help you better understand yourself and why you feel this way.
There are no guarantees that any relationship in your life won’t end, and that’s what makes them scary. I was in similar long-term relationships at the same ages as you that ended for the same reasons. They were both great people, but those feelings of love weren’t there anymore. It sucks because you still love and care about this person, and it feels terrible to hurt them, but you’ll only hurt both of you more by staying in something you know you don’t want anymore. Being single is always better than being in a bad relationship.