There is nothing wrong in loving somebody who is already taken because, in the first place, you didn’t choose to feel that way towards them. It is what your heart dictates. And when the heart speaks, most likely, nothing in your system can win in it. Sometimes, it is more dominant rather than the brain.
What’s wrong is when you keep on pushing yourself to that someone to love you back. that’s the kind of instance that brain should rule over the heart. If you will insist that phase, you will just ruin a good relationship. Yes, I understand that everybody who is in love wants to be loved back. But in that scenario, you should also think of the other people around you. If they both love each other, what’s the sense of ruining their relationship? if you love a person, you will prefer his/her happiness rather than your own. If s/he is happy with someone else, let him/her be.
Moving on is not easy but that’s the only resort you have if the one you love doesn’t feel the way you do. Just wish happiness for the both of them. In God’s time. you will meet the perfect one for you in whom you will find an eternal happiness more than you can ever imagine. 🙂