The key thing here is to remember that <i>you</i> aren't selfish. You <i>behave</i> selfishly, but that's not <i>you</i>, it's just how you <i>behave</i>.
That distinction is crucial (which is why I <i>italicized</i> it. <i>Twice</i>.
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Armed with that knowledge, you now have the ability to recognize selfish behavior, catch it in the act, and interrupt it. At first this will be very difficult for you--it'll look like the only thing for you to do is be and do the things you're being and doing. But as you practice (and it DOES take practice) you'll get better and better at intervening in your automatic actions and ways of being.