Selflessness

Why do we keep taking ourselves out of the equation?
What is the purpose in our lives if we don't live them the way we want to?
How can it be that I know exactly what I want in life, but i'm still not able to make it count. I behave in ways that pleases other people. I put myself in a box, labeled by others and for what purpose.
A friend gave me some very wise words today: "Selflessness is one of the most admirable traits someone can have, but a certain level of behavior rooted in a healthy respect for oneself is necessary."
Hearing this, I totally agree with her, but now comes the hard part: How will I be able to incorporate this in my every day life? It is not in my nature to be selfish and it can be a very bad quality in a person if overdone, so where is the balance?