Sartre and Psychology?

Now I know that psychology was branched off from philosophy. I would still like to say that some of what Sartre said struck me as psychological. When he was explaining how existentialism is optimistic and not pessimistic, the example he used to describe it, reminded me of things that I had learned about in psychology. People attribute their own failures to external factors and their successes to internal factors. In terms of the example he gave, he showed how people will comfort another by saying they are this way because they were born this way or the environment they lived in made it so. I think Sartre touches on how this is not the case. Although I personally believe it all plays a role, Sartre comments that it can be changed. And on this, I do agree with him.