Hi Charlie - I'm 65 now, and I still sometimes wonder about the path my life has taken and if I'm fulfilling my purpose.
Not only do Aspies 'mature' at a different rate than NTs, but we also tend to psychologically consider and reconsider, and reconsider, a lot of what occurs in our lives and how we respond to it. NT maturation rates place adulthood/maturity somewhere in the mid twenties, with us it seems to occur in the thirties or even later. Our prolonged rate however, also seems to create a deeper and wider sense of consciousness (at least to ourselves, if no one else ) I've noticed this deep internalization and fluidity in the 'sense of purpose' to be rather common amongst us. So, you should consider yourself quite normal.
NT psychological profiling tends to label this sort of thing neurotic, but that would only be correct when measured against the whole of humanity...amongst your peers you are right about where one should be on the maturation scales. Actually, a lot of NTs also are considering such things in their early twenties - though many NTs would be scratching their heads at your 'problem' - it's totally outside their socio-cultural norm.
No one can tell you the answer, or give you your purpose with any precision...though a lot of social institutions like religions try to, as well as unscrupulous individuals who can sense your inner puzzlement and attempt to take advantage. Since you work in a school that focuses on autistic education, you might have access to people you can take counsel with - but ultimately, this is one of those core life issues that can only be accomplished by oneself. Also, as JuliaW mentioned, a sense of one's purpose can change through your lifetime.
It's sort of like riding a bike. Keep at it and you'll find your balance and start moving forward. And like riding a bike, as you move along you might find your path and goals change along with the landscape.