I like to think of it as being on a spectrum. At one end, there are doormats who let everyone walk all over them. At the other end, there are narcissists who have zero empathy or compassion at all. Both extremes are bad.

One should therefore work at being in the middle, balancing self-protection with empathy for others. There's not really a way to stay EXACTLY in the middle, though, as one is going to lean more toward self-protection sometimes and more toward empathy at other times. But over all, it's best to hover around the middle and away from the extremes.

The thing is, for someone who's accustomed to being more toward one of the extremes, moving in toward a healthy centered balance is going to FEEL unbalanced for a while, because you'll think you're moving into DANGER NARCISSIST TERRITORY even if you're still on the doormat side of the center. That awkwardness is normal for a while. It's also normal to overcorrect a bit for a while and occasionally be overly selfish, and that's ok every now and then, too, because you can't really find center without venturing into the other half of the spectrum from time to time.

I imagine that there's probably a good motorcycle analogy here, really, but I don't ride, so ignore if this is unhelpful: Leaning way too far left or way too far right will make you fall over and keep you from moving forward, but staying balanced requires microadjustments right and left as the course changes in order to stay centered.

It only looks selfish to selfish people, who would rather you see to their needs before you take care of yourself.

And "being more selfish" is not in any way the same as "being selfish." I can become older with being old, or thinner without being thin, or more calm without becoming calm.

Absolutely! Your oxygen mask analogy is perfect, and once I was able to apply that lesson to my own life, I was able to straighten my own shit out with a great deal more tranquility.
I was (am) a classic codependent. I used to keep myself quite busy taking responsibility for other people's messes, and cleaning their shit up. The level of involvement I allowed myself to achieve with certain family members (I'm the oldest of four, I've got issues) was crazy making. I was utterly consumed by my "selflessness".
One of the things that I did to cure my over-involvement in the messy lives of others was develop a layer of selfishness. Selfishness for me involved setting boundaries: not answering my phone in the middle of the night, not giving all discretionary spending to someone else to pay a bill, not spending my free time ferrying people to this clinic or that. By setting boundaries, I freed up time and mental space (and cash! huzzah!) for myself. I needed it. We ALL need it. I realized that all the crap I put myself through "for the sake of others" was ultimately useless (they weren't getting any better because I was depleting myself on their behalf) and unnecessarily taxing on my own person. I needed to learn some selfishness. And now as a result of this newly found selfishness (which I have been implementing over a period of two-odd years or so), I am healthier, stronger and happier as a result. I am now better able to put the oxygen mask over someone else's face, because mine is securely fastened.

I am now a more selfish person than I was before. That is a good thing. It does not mean that I am a selfish person.

Since the mid-20th century, the word 'selfish' has picked up an undeserved negative connotation. This is somewhat similar to the difference between an egoist and an egotist.

I'm all about the quotes these days, but I've been thinking about this one:

"Sometimes you have to be selfish to be selfless."
-Edward Albert

You can't really give selflessly to others unless you are a fully realized being with a sense of self love. Sometimes being "selfish" is the most selfless thing you can do, because it truly enables you to approach the world with an open giving spirit, unencumbered by your own hangups, because you were "selfish" enough to take care of your own needs so that you had the energy to give freely to others.

Obviously you grab all the oxygen masks you can reach and rent them out to people around you. No such thing as too selfish.

Do what's good for you,
Or you're not good for anybody.

-B. Joel

Amen, Doohickie!