We can't get away from the fact that our life is our own responsibility. Especially concerning our health.
That can become such a struggle for so many of us (myself included) when interacting or intertwining our lives with those of the people for which we most care. Perhaps this is the greatest challenge for those who self-identify as care-takers in relationships. I've put myself in that situation in most romantic relationships as well as close friendships. You put their needs ahead of your own (which of course sounds wholly altruistic) but can be taken to the extreme for persistently sacrificing your own health and well-being for the entirety of the relationship, and not just limiting such measures to dire periods of time. This speaks largely to knowing what healthy boundaries are within different relationships.
I'm sure if you give it some thought, you can find examples of both in your own life. I can't speak for you, but an area which I can find rather challenging is when spending a lot of time with friends. I'm the kind of person who derives much of my energy and happiness out of my interactions with other people. So quality time makes a big difference in improving my sense of well-being. An example of the positives of that would be that I feel energized and accomplished when making other people feel good. But a negative to that is I will make too many sacrifices of what I know I need to do to take care of myself, in order to regain a certain balance within relationships. Whether or not the fault is my own. Again, that's a boundary issue.
What does this have to do with a fitness post? Understand that if you feel you fall into the above profile, many times the people around you don't. Does that mean you have to change other people or eliminate individuals? Of course not. But we do want to have an understanding of the fact the many people will not, or can not prioritize others the way you do. Therefore, do not consider it selfish to have your own health (and maintenance of) as a top priority. Except in extreme situations, be aware that the food you eat, your workouts, supplements/medication, etc should be high priority when contrasted against certain (not all) needs of others. I don't mean going to the extent of only ever eating your own foods. Limiting all social interactions to the point of never going to a restaurant or traveling because you won't be near a gym or have your prepped meals. Or any such fear-based decision.
But I have seen, both in mine and clients lives, skipping multiple workouts, meals, supplements, etc for the sake of putting all the needs of another person or relationship above your own. I can often fall into the trap of assuming the other person would have already made the allowance of me needing to take my medicine at a certain time, sleeping a certain hours, or having room in the day for a workout. I'd then fall into the trap of being hurt or feeling neglected. But do you see that if I'm the only one in the situation who's the natural care-taker, I really set that trap up for myself? It doesn't mean other people are self-absorbed, they just operate differently. So in order to make sure my needs are met as well as theirs, assertions have to be made on my part as well as reinforced and upheld. This is how we create balance and healthy boundaries. This is a huge part of maintaining ones own health.
Hopefully that can also help clarify for us that it is not "selfish" to go to the gym for an hour or bringing some of our own food with us on a vacation. But just an aspect of making sure our own needs are met.