Only you decide what your purpose is.  If you choose to denigrate yourself due to an action or a choice you made that makes you feel you are worth nothing, then you need to eek mental help.  Its a mental health issue.  Why cant we go back to the jack of all trades mentality where we can be good at a lot of thing, and if something doesnt work out, we move on to the next

What I am trying to say is quite looking at the glass is half full, and take charge of your life.  When something fails, learn from your mistake and move on