What are you told during the emergency procedures on a in flight demo?
Put in your oxygen mask first and then assist others.
Why? Because if you lose consciousness your no use to those around you.
Similarly, if you run yourself into the ground and continually put others before you eventually you will get burnt out and of no use to anyone.
It’s not selfish to put yourself first everyone once in a while. The happier, healthier you are then the more of you there is for you to give to your loved ones each day.