Why don't I tell you life's purpose to me, then my life's purpose, and perhaps somewhere in between, you may find something that gels.
I feel that I am an instrument of Something Higher, called by many names including 'God.' My purpose then is to glorify, praise, adulate or worship this Source through the manifestation of His/Her glory. Actually this can take many forms in so far as I see whatever I do as an act of worship.
Your question is marginally different, as you are really asking what you are meant to do. A God-man would tell you this, but they are very hard to find and can literally be counted on your fingers. So your next bet is to make yourself receptive to Grace, via service, prayer, gratitude, meditation ... the things that would expand the Heart and bring down the twins of self-effort and God's Grace.
My own life's purpose is to serve. I was doing this unconsciously, but when I became conscious of the Divine through inner experiences, it made it all very clear. I have done this through nursing, in a shop and in being a policeman in early life. I have served consciously for nearly 35 years.
Actually, you can dive deep within and learn to listen to the inner voice. The Inner Pilot tells us all, but we have to listen in Silence and act accordingly. Each one is unique; each one has his/her part to play in the scheme of things. God is the life-tree and we are His/Her branches. The Soul needs us to manifest the Divine; we need the Soul for our Salvation/Realisation. God speed!!