If the guy whom i love very much is lieing to me and thn also i cant ask why he is doing so??

i love one guy and he also loves me..we are in love since last almost 3 years..he is having two numbers..but whnever i ask him bout his another number..he starts behaving rudely..i know well that he is talking with another girl..then also i m feeling helpless to ask him..i dont want to loose him but at the same time he is cheating me..what should i do??