Some people are so altruistic that a share of healthy egoism will only benefit them. In order to develop self-love and learn to think about your own interests in the first place, you need to work on yourself.
To your desires
In order to develop healthy egoism in oneself, one does not need to suppress one's own desires. Instead, go meet them. Think more about your dreams instead of serving the fulfillment of strangers.
Start changing with little things. Every day, think about how you can treat yourself today. Let it be little surprises. The main thing is to feel the taste of pleasure, to understand how nice it can be a manifestation of self-love and concern for one's own person.
Maybe you have already atrophied the ability to desire something. Here, you are watching others who are striving for new heights, but nothing turns upside down in the soul. Then imagine your own life in a few years. Think about what it should ideally be. This exercise will help you determine your life tasks. In the future, you need to follow the intended course and not deviate from it for the sake of others. Divide large tasks into smaller ones and work on the progressive achievement of the goal.