As a Christian, because of Christ, I have a good self. In order to walk by the spirit I must acknowledge the good self (spirit, identity) that God has given me. My spiritual identity is in fact my only identity in God's eyes. Thus it is my only identity, it is my only "self." If you must talk about selfishness I think self-centred is a better word to use than selfish. Jesus had the greatest self ever but He was not self-centered. The opposite of self-centeredness is love for others. If I focus on self to the exclusion of others that is a harmful thing.
Jesus knew who He was and the importance of what He came to do. If He was not confident that He was the Son of God everything would have been in disarray. Likewise we must have confidence in the fact that we are His children & confidence in what it means to be born of God. The idea that I should degrade myself is sick, satanic, & useless (Colossians 2:18,23). For the Christian a focus on our true self (our only "self" as God sees us) is a good thing and is actually a focus on God. If I rejoice about my identity in Christ it is all because of what Christ has given me & done for me.
An amazing thing to do is to also rejoice in how God sees other people, about their true identity in Christ especially when they treat you poorly. We who are in Christ are equally loved, equally holy, equally righteous, & equally perfected whether we can see it or not. To see someone through the eyes of Christ disarms your negative thoughts and emotions towards them. If we have an awareness of sin, that is if we count sins & label things as sin, it is impossible for us to see anyone, including ourselves, as God sees us. God has taken the world's sin away. He has removed it from people so now He does not regard anyone according to their flesh based actions. God knows this is best for us too. Not regarding people according to their flesh based actions changes how you treat people. You cannot judge or compare by pride. You simply love because you are loved & they are loved. God's love is like butter, it is better when spread.