if the guy is in love with someone, and he's already committed to that person... i think it's best not to tell him. although most of the time, it's true that telling someone how you feel can set you free --- some truths are better left unsaid. how would you feel kung ikaw yung gf/bf di ba? would you like it if someone would approach your man/girl to tell him/her na mahal sya nung person na 'yon?
besides... if a guy really is into you --- 'di mo kailangan tumambling para makuha ang kanyang full and undivided attention. if he feels the same way, he will find the courage to tell you that he does. and you don't even have to compete for him dahil kahit na hinahabol-habol sya ng iba (kumain man sila ng apoy, magpagulo-gulong sa bubog at tumawid sa kumukulong swimming pool ng mantika)... sa'yo pa din 'sya lalapit kung ikaw talaga ang gusto nya.
you're a cool girl, fated. just sit back. the right person will come along in time.