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You could take an entry level course in each of those fields at a community college or university and see which one inspires more passion. If that is not feasible due to time or expense, find someone in each of these two fields who would let you shadow them for a least a day. Spend a day with them to see what the daily life of a nurse or teacher would be like. This may help you decide.
Perhaps better yet, try to find a career that allows you to combine both of these fields....like a health educator....and again, shadow someone in this field. Hope this helps and good luck!