There is perhaps nothing as damaging than unrequited feelings for another person. Feelings are not like thoughts and it is difficult to rationalize with them. You cannot decide to like someone just as one cannot decide not to like a person. Feelings are pre-rational.
You can control your behaviour and act honourably or dishonourably. It is always possible to say nothing when one wants to speak or to lie to save someone else's feelings. Yet, you cannot 'convince' someone to like you. Nor should you be too disturbed when someone you do not really care much about, expresses unwanted feelings. All you can do in such circumstances, is to act with respect and give a polite but firm 'no.'
When you feel strongly for someone and they do not return these feelings, it can be very hard to deal with. The first response is to feel that one's pride has been undermined. This can then develop into feelings of injustice. It takes a lot to leave oneself vulnerable and risk rejection. This is why repect is of the ultmost importance.
One cannot make oneself unfeel the other person by making a decision. When someone says, "It is time to move on..." it is never that simple. While such advice is often given with concern from a friend, one cannot simply wish the other person and their memory away. All one can do is live with the pain until the feelings die a natural death.
To try and force this issue can lead to damage. One can drink oneself into an unfeeling state. Or one can learn to hate themselves and then the beloved can become a victim of nasty remarks or worse. Socially, the best one can normally do is to remain silent.
But this is when the therapeutic value of art becomes apparent. To write a novel, love poem or song- to paint or make a film is an activity closely connected with bereavement. One both learns to remember and forget slowly in an act demonstrative of love.