Do self centered people realize they are being selfish ? or focus on what they want and how they want?

No they can not, as the saying goes "love is blind" they are so blinded by self love they can't see.

Nobody else matters, so why should they realize it?

No they are narcissistic and cannot see and admit their own faults...which they will project onto other people who don't even have them...have a sister that way.

Everyone is selfish to an extent but there will always be those who think the world revolves around them. They must realise it to a degree but choose to ignore it!

Some are completely oblivious that other people have feelings and don't see themselves as self centered. There are many more, however, that can see outside themselves until they find out they are surrounded by stupids and uglies so they crawl right back inside themselves.