Lol. It's funny how you said that people who give a sermon about it make it seems as if finding one's purpose is easy to come by.
Truth be told I have also been there.
It got to a point I was confused about life and I usually ask myself how I can I discover my purpose.
I remember praying about it on different occasions but after a series of trials and it felt as if I was forcing it. I let it be and just follow the directions I believe God is directing me into.
So, my dear, I don't also know how people discover their purpose and how they know for sure that this is their purpose.
They say the Holy Spirit guides and directs and this is why I believe the Holy Spirit is the one guiding me through all things.