Spend the next 7 days doing something very, very selfish each day. If you're having difficulty feeling good about being incredibly selfish, then it's time to go on a scavenger hunt. Make a list of 7 things you really want, but haven't been able to let yourself have. Then, one a day, go grab these things whether they be tangible or intangible. Say no, just because you feel like it.
Selfishness is a muscle that needs developing.
The easiest place to start is to say no. If you can't say no, get a coach to show you how and support you to say no easily. Saying no is a learnable skill that pays dividends for a lifetime. The real value of becoming selfish is to give your gifts room to develop. Take what you feel you need, even if it means that others won't get as much as before.