While it’s possible that selfishness is involved in some way, reducing it to selfishness alone is unnecessarily judgmental. Examining the source of that selfishness would be more helpful for someone like OP who wants to understand/change.
Selfishness can come from many places. The need for self preservation is not inherently bad or wrong. But when it surfaces in a context like this, it provides an opportunity for someone like the OP to understand themselves better. We’re not competing for the last scrap of food, and so these instincts don’t always serve us well.
I grew up in a traumatic environment. My self preservation instincts are highly attuned. This led to some default behaviors that some would label selfish. I’m now one of the most helpful people I know, as shared by those around me and I find great satisfaction in it.
This didn’t come naturally, and I had to cultivate the mindset. I wasn’t selfish because I didn’t want to help people, I was selfish because I was afraid of my own failure and the perceived ramifications of that failure (ultimately: loss of income and starvation).
I eventually learned that being helpful can be one the most self preserving things I can do, because it helped me built a support system that now stands strong.
Selfishness is a symptom, not necessarily the cause. It’s worth examining, but saying “stop being selfish” could be as useful as saying “stop being depressed” if the source is something deeper.