Without reading the other responses, first resposnse is to say... what it means to be selfless is
Being a betrayed spouse. Offering the gift of R. Being alone in your triggers and trauma because that's 'yours' to deal with, alone. Trying to be happy again when you've literally lost years of your life by being kept in the dark, again it's your job to be happy even though you didn't cause the pain and it would be feasible for the one who did to try and bring you happiness because logically it's possible if they can bring you pain.
Being selfless is being a good parent and not telling your children the truth about your WS so they can look like the good parent. It's defending your WS to your family and putting up with theirs who don't support you even though you were wronged.
It's being the perfect saint like forgiving, loving and gracious spouse to the one who thought it was acceptable on some level, to give you an std and steal your choices. Because if you don't then strangers who are supposed to understand and those who don't, will criticise you for not being exactly that.
It's giving credit to your WS for being a decent human being after being an indecent one to the one person they were supposed to be good to, while only getting credit for being perfect. Because your WS is 'trying' nevermind that you're 'trying' harder because you have more to deal with and didn't ask for any of this.
It's forgiving the ap or you've got issues even though it's normal to have issues. It's not feeling like plan b or the second choice even though that defies your natural response or otherwise you've got issues. It's finding confidence (sexual and otherwise) from nowhere and no one and everywhere but, the one person who should provide that beven though logically it's fair for you to need that from the one person who you have a romantic and sexual relationship with.
It's being alone in your mind movies while making love to the one who just learned that it's wrong to give your spouse an std or cruel to have sex with their AP in their marriage bed. While other betrayed spouses tell you to 'just stop thinking about it and be so present'even though thoughts don't work that way and feelings certainly do not. While everyone tells you that marriages can't survive without aex so you 'have to' even though the same doesn't apply to the the marriage not being able to survive with the std they gave you.
It's giving your betrayed spouse another chance even though you're the one with more to lose, because despite having to learn that you'll 'be okay no matter what', if they betray you again it might actually be worse than the first time and you'll have to recover from that before you can be okay. Meanwhile they start a new relationship soon after and treat the new person better, which is fine because it's okay for WS even on their first posts to be told they should work not themselves not for their BS but for themselves and possible new relationships, before they've even atonned for breaking the current one and the current person.
It means accepting that your partner is a liar and you'll never really know if they're being authentic with you and all the phrases and positive stories in the world of SI and out of it won't give you peace, because you should not have live feeling that way. You deserve better than to question everything and yourself and no self care will change the fact that you've been treated cruelly and made to deal with it predominantly on your own, while your spouse's main job is to do what you've been doing all along- being faithful.
It's all of this, and knowing that your life is diminished, but trying to keep hope that somehow it's better this way or that you can have the same amount as happiness that you would have had otherwise, even though that's not really true for you. But you can't feel that way because 'others have healed and they say otherwise'. Because it's not okay to admit that you feel your life has been ruined by the one who claims to love you, as if you wouldn't know, even though you're the only living it.
It's being the one that gives more and gets less out of the relationship, because no one wants a 'better relationship'and 'spouse' at the expense of their happiness, when they are still dying inside. Because they still can't trust that they are number 1. Because a marriage can't be 'stronger'than before if reconcialtion happened the before but won't happen again. Because the fact is if you love someone then you care about their happiness and well being and don't do anything to jeopardize that.
Which is why betrayeds are selfless. Because they have to be held to that standard of love. Have to give that love while knowing they will never ever ever get it back from their WS.