What do you call this?mental disease?

[singasong]

hi,I was wondering what do you call this situation-you like somebody a lot but dare not tell him/her nor even talk to him/her a word. He/she is in your mind all day and all night and drive you crazy. How do you call this? Is it a mental disease? What could you do but to escape from him/her?

Have you had ever run upon this situation? What did you do to tackle it?