There is a great difference in these two concepts, and a very different energy/ feeling associated with each. Being centred in oneself is being truthful to oneself, acknowledging oneโs beliefs and desires, and following through on obtaining/achieving them. Seems basic enough: you know who you are, what you want, and youโre going for it.
The point at which being centred on yourself and being selfish occurs when going for it has negative effects on those around you. Are your actions and words intentionally hurting others? Perhaps an example will help. You have a passion for music and have a desire to listen to it at 2:00 am. If this is what you truly want to do, then go for it, but a selfish individual would blast the stereo without any concern about how this will affect others within earshot. A more reasonable solution would be to put on those headphones.
The desire to hear the music is satisfied, and there are no negative effects to those around you. Truly just be aware of the consequences of your action (karma). If we could all be just a bit more courteous to each other… what an impact that would have.