Sometimes it’s not you, it’s her:
Selfishness is a human trait, so it is a given your wife has some selfish areas. It is also human nature to hide our selfishness from friends while allowing it to leak out with our family. Odds are you get more of her selfishness than anyone else does. Selfishness is worse when we feel stressed, overwhelmed, or empty. When we are doing well we manage to control selfishness; when things are going badly selfishness can become a real monster.
I mention all this because when and why she is selfish matters. If she is always selfish with you, you have a serious problem and you need to get third party help ASAP. If her selfishness waxes and wanes, she is normal and there is hope she can grow up and do better.
Once again, leading by example is your best play. Selfishness begets selfishness; selflessness encourages selflessness. Work on being less selfish, and talk about the process. Ask her for feedback, and ask her to let you know when she feels you are being selfish.
Unless she has invited you to let her know when she is being selfish, saying “you seem particularly selfish today” is probably a bad idea. Instead, ask her about how she is feeling. Ask her if she is upset, tired, or stressed. Your questions will make her aware you see something wrong, and will help her see herself better.
Work to understand what brings out her selfishness. If being tired makes her selfish, work at helping her not get too tried. Reducing the situations that trigger selfishness is not a solution, but it will help while she grows.