I need advice?

ok what do you do when you love someone but there already happy with somebody else? and it might change something between you if you tell them but, you just cant because it might mess up their current relationship that has made them the happiest in a long time. i want this person to be happy just as much as i want to be with them.. and im not so sure if theyll be happy if i tell them and i will probably make things really uncomfortable between us. i think about this person night and day.. there the only person who can make me truly happy.. i didnt know i could smile like that till i met this person.