I was told once that there are actually three people in a (traditional) relationship, the two people and circumstances. That sometimes circumstances have a say in the relationship, even if you don’t want them to. And I responded that though that is true, you don’t just go through life letting things happen to you. You choose what happens (obvious examples excluded). That, to some extent, you can make the things happen that you want to happen.
Of course, sometimes it’s more difficult than others.
But it’s also true that the two people in the relationship are not the same person. This seems obvious, until you realize how much you instinctively think that other people think like you. They don’t. And what’s best for one person is not always best for the other. And what is possible for one person is not always possible for the other.
Even in circumstances where you are very much compatible, you are still not the same. (Besides, that would probably be boring.)
Sometimes the other person already made the choice, so it seems like you have none. And sometimes your choice is between a rock and a hard place. And that’s life. But you can choose how to respond.