The pronoun's referent governs its form. Does the calling belong to "I" or to "a multipotentialite"? (Of course those words refer to the same person, but I mean grammatically.)
If the pronoun refers to "I", then it must be "my". However, some people who consider "a multipotentialite" to be in the 3rd person (see below) might find it strange for a different person to find "my" calling.
If the pronoun refers to "a multipotentialite", then we have a choice:
If "a multipotentialite" is considered to be in the 1st person,* then it should still be "my". (Prescriptivists tend to favor ascribing the same case to a predicate nominative as to its referent.)
If "a multipotentialite" is considered to be in the 3rd person, then it should be "his". (Ascribing the 3rd person indiscriminately to predicate nominatives is very common in practice, both in writing and in speech.)
To sum up: Either is justifiable.
*Some people do not like using the term "person" with noun phrases. Nevertheless, my meaning should be clear.