Purpose is the very thing that separates a mundane life from a fulfilling one and something which is constantly evolving. It is very rare that we all have one purpose that lasts a lifetime. Purpose comes and goes, but is the very foundation on which our happiness can be built on in any given moment.
I’d like to make a suggestion to the psychologists. Take a step out of your own boundaries and text books and conduct some research starting now with people who apply techniques to change the way they think and feel about themselves, whether they have low self esteem or not, then after several years, publish the results. You might learn something.