"Doesn't mean you're right :p"
Certainly not! But you may be pleased to learn that part of my philosophy is that I might be wrong and should constantly question it.
"It is not based on the belief that your own happiness is paramount. "
Yes it is. Even if you disavow this belief, this is the motivation for your actions.
I am for instance not at the center of my value system, but I at least acknowledge that my value system is what it is only because it satisfies me.
You may be fretting that if you admit that all action is selfish, that altruism and sacrifice lose meaning. Not at all. Like I said, it's just calling the sky blue. Admitting the truth about the motivation for your actions does not prevent altruistic behavior in any sense. It is just a statement of fact. Altruism is still a worthy thing and rightly honored by others. People tend to think altruism is good because most people value more than just themselves.
But you should remember that although most people do assign moral worth to more than just them, they only do so because this is what makes them happiest, or so they think.
I cannot be anything other than egalitarian. I cannot countenance the idea that a human being is less worthy of happiness than me. Thus, because believing anything other than what I believe about egalitarianism would bring me great moral pain (a kind of unhappiness), I have my belief. It is also rooted in empathy, but empathy is nothing more than the ability to feel for yourself the emotions you see someone else experiencing. If I had no empathy I would probably not give a damn about human beings other than myself.
Selflessness exists, of course. I am not attempting to undermine it. Indeed, I would be very upset if I came to the conclusion that there is no such thing as altruism or self-sacrifice - to me these are almost holy - more worthy than any other state. For me it's like an instinct to see it this way but there is an obvious reason too - I know that biologically I tend to favor myself above others - it's what the Darwinian beast which is certainly part of me wants. But this tendency is inconsistent with my beliefs, which are egalitarian and by extension self-sacrificial. Thus when I successfully do something self-sacrificial, I feel morally enriched - it is the joy of being generous, for example.
Thus by denying myself I stumble serendipitously upon happiness. I have seen this occur and take it as yet another reason for my belief in egalitarianism and self-sacrifice.
I never denied there is no difference between a selfless person and a truly self-centered person.
However what I did imply is that outside of any moral system (the details of which are dependent on what makes those who hold the beliefs happy), it is meaningless whether someone is being selfish or selfless. Both are acting on the same motivations, but one chooses a different path. Neither can be said to be different, though, except perhaps from a Darwinian perspective, which is amoral (which means outside of morality, does not mean has a bad morality).
So now, I hope, you see what I mean. Within an egalitarian system like my own, selflessness is a great thing, and a real thing, and something to strive for. But within a moral system where all people are a means to an end for me, selflessness is folly.
So that's the framework. What you have to do from there is try to engage people on their level - learn what their current moral system is, and appeal to aspects of their humanity that you believe they share with you in the hopes of converting them to your moral position, if, that is, your moral system places an importance on other people coming around to your view, which mine does.