Keith Urban 's a heavyweight "Fighter" in the music industry ... and, he says the toll of being a big-time star is all-encompassing -- even when it comes to relationships. The singer-songwriter sat for an interview with "Entertainment Tonight" months before his estranged wife, Nicole Kidman , filed for divorce ... but it seems he was already reflecting on all the sacrifices he's had to make for the music. The interviewer asks him about financial and physical sacrifices ... and, KU offers up "