Selfishness???

Selfishness. That's a term we use so easily with children. "Don't be selfish, share your toys." "You are being very selfish right now." The list could probably go on and on. However, as an adult I find it much harder to determine when I am being selfish, and when I am just trying to have a little backbone to keep someone from walking all over me. Many times in my life I simply give in to decisions or don't give my opinion on certain things because I don't want to seem selfish. However, what is the line? Where does it go from giving an opinion and wanting your say to matter to being selfish? The Bible says in Proverbs 18:1, "An unfriendly man pursues selfish ends; he defies all sound judgment." It also calls selfish ambition an act of sinful nature (Galations 5:20). Phillipians 2:3 says, "Do nothing out of selfish ambition or vain conceit, but in humility consider others better than yourselves." I try to do that to the best of my ability. I try to put everyone else's feelings above my own. However, sometimes it hurts. Sometimes it causes pain that may have been avoidable. Is this what we as Christians are supposed to do? If so, how would anyone ever accomplish anything? If 2 people are both Christians and put each others opinion first, how does a decision get made? I realize I'm rambling, and probably not making any sense. I may come back to this later and try to explain myself more effectively.