Some say it's better to have loved and lost than to not have loved at all, but this isn't always the case. The hardest thing to do is to love someone who doesn't love you back, especially if you're in a relationship with that person. If you've never been there you're lucky, being with someone who doesn't love you back is the worst position to be in.
It seems like no matter what you do, or say, nothing will change how they feel about you. They may tell you that they love you, but you can feel that isn't real. In the other case, perhaps the worse case, they never say they love you. When you say you love them they respond with "I really like you," or "I'm not at that point yet."
Something is holding them back from really opening up to you. They may talk about their feelings but they never break down, you never get to the really deep stuff. It's hard and honestly I'm not even sure if it's even worth it. You open every door and pour out everything because you love them, and you get nothing in return. Just empty words and broken promises.
If they don't love you back they obviously aren't going to put in the effort, so the whole relationship is you running around in circles trying to get that love. It's a self deprecating process and at times you might not realize they don't love you back until it's over. This is the worst case scenario. The entire time you're together you say it, they say it back. You go through so much together, but you're always trying too hard to make them happy. They cast doubt about the relationship like a never ending storm, and that's how it feels. Like this storm will never end, and that the sun will never shine. Yet you power through because, at least you think, you love each other. When the relationship finally does break down you can see it, they never actually loved you and it hits you like a bomb.
When you get out of a relationship like this, it all seems so worthless. All the time, energy and love you put out there. All the support you gave to them and what did you get? A broken heart and lesson learned. But don't let this ruin you. Not every relationship is like this and someday someone will truly love you, more than you could ever imagine.