As someone who considers selfishness a virtue, it's no surprise that I believe everyone would be better off if they put themselves first. I'm all for helping others, but you can't do that if you're not helping yourself first. How could a dying, weak, malnourished, uninspired person possibly make anyone else's life better?
There's just one gist: in order to put yourself first and do what's good for you, you must first figure out what it is that's good for you.
This could (and probably will) include work, relationships, hobbies, lifestyle choices, physical health, mental health, thought patterns, and routines. To start with. The list is infinite.
So if something isn't working, what isn't working, and why? How do you fix it?
If you're miserable, why are you miserable? How can you turn it around?
Is your work meaningful and are you inspired to do it? Do you do it enough?
Are you happy in your romantic relationship? Do you like your family? Are your friends lifting you up or bringing you down?
How is your overall health? What do you think about all day? Do you have detrimental or beneficial daily routines? How can you impact them?
One way to check in with yourself about what's working and what's not is having a journal and logging in your challenges and successes. You can refer back to it whenever you're unsure if something you're considering is a good idea or not.
Another idea is to imagine the perfect life in your head and see what it looks like.
Another way is to listen to your gut feeling whenever you're experiencing something specific.
Another hack is to see if doing something / eating something / seeing someone drains you of energy, or energizes you.
Then you can also scan your immediate surroundings and the media you consume daily for negative triggers - ones that you know, on some level, are bad for you.
Whatever the way - you can use all of the above or invent your own - figure out what's good for you, then do that.
Then, and only then, can you look around and see if you can have a positive external impact.