I like to pose this question to clients who are immersed in "caretaking" behaviors with their partner. If you are sitting next to a child on an airplane and the oxygen masks drop because of an emergency, whose mask should you put on first. Very often, "caretakers" will say "the child of course". On second thought, it usually becomes obvious that putting on one's own mask is the first order of business as one's own breathing is essential in case things don't go as expected.
In relationships this translates to-First love and nurture yourself so that you have energy to give to others. If you do not take care of yourself, you will have very little to offer those you love. Yes, this is selfish, but without self care, we all gradually lose our vitality and ability to relate.