If the person who you like doesn't like you, then even if people from all over the world like you,you still feel lonely.
应该没错,采用吧。
If the person you like doesn't like you, so even if the whole world loves you, you will still feel lonely.
fall in love with sb 爱上某个人 (避免前后重复 使用这个词组更妥当)
if the one you like doesn't fall in love with you, then you still feel lonely even if the whole world likes you,
If the person you loved doesn't love you, then you still feel lonely even all the world love you.