For me, I'm thinking of the reverse circumstances. A memorable pain for me was one person in particular, years ago, - - I remember when I realized his love (infatuation?) for me had changed but mine for him hadn't.
Regardless, I think your scenario and the "reverse circumstances" scenario are both potentially painful.
Has happened in my youth. Trick is to let them down lightly.
Converse also happened. Lass was quite nice about it and introduced me to some of her friends. Fact was she had to admit she had a real boyfriend up her sleeve.
Not now but I've had that. I don't think it's painful. I honestly think in any relationship, there is always one person who loves the other one more. It's rarely an even situation. The trick is in how to handle it.
That is natural. In no relationship do we "love" "equally". If he (she) doesn't like you then he doesn't like you and it hurts but mostly because what you hoped for did not materialize. Your life is not over so let him go or rather let go of him and live your life.