It's an amazing feeling, yet, an equally terrifying feeling to feel that you truly KNOW your purpose in this life.
I, too, can say that i feel that I have reached this understanding. So i can't tell you what you are feeling and why, but i can only speak from my own experience, which i guess you can take as an example, something to ponder.
I am entirely convinced that I know my purpose for being on this small little planet, in whatever lifespan I am given to walk it. However, i feel as if it is no small purpose, and so, i absolutely agree with you when you say, "Is it possible to feel unworthy of what one thinks they are meant for?" I would say, absolutely, YES, this is possible. I am continuously overwhelmed with this feeling.
I feel as if a have an enormous burden on my shoulders, and that I am not worthy or able to undertake this task for my life. With that, no matter how much i seem to accomplish within this area in which i feel i am destined to be, i always am overcome with feeling like i am not doing enough, not going fast enough, that i want to do too much, too soon, and i feel perpetually unaccomplished. sometimes i just ask myself, "why me, damnit?"
sometimes, i wish i had no drive, passion or purpose, so that i could sit back and relax, take things as they come. but those feelings are fleeting, because reality sets in and i realize that just isnt me.
I am also a believer in dreams coming true, it has happened for me in many respects already. but the thing about dreams coming true, is that no matter how well you plan things out and push to achieve a goal, actualize a dream... if it happens, it will NEVER happen in the way you imagined. but.... this is the BEST part. if life were entirely planned and followed accordingly, it would be boring and pointless... anytime you think you have everything figured out, life has a funny way of proving otherwise. but this is why you must enjoy these small or sometimes large changes as they come... its not about fighting change, but grabbing it by the reigns and steering it in the right direction.
so for you, congratulations in discovering your purpose... don't let it overwhelm you or become a daunting cloud over head, but rise up with it, embrace it, and enjoy the ride.
ultimately, we don't know anything until its all over. so, do what you feel is best, and i guess you'll know if you were right when you're dead. haha.