Q: Are we born with a predetermined task that we live with and that we willingly (or unwillingly) perform, or, as the Ari explains, can we attain a soul by labor, effort and prayer?
A: It is not up to us to decide what soul we will be born with, what attributes we will have, and with what task or role we will come into this world. From childhood, we begin to unconsciously advance towards our task, through our environment and the circumstances we are given.
If the task deals with connection with the Creator, then at some point if we have matured through past lives, we will begin to search for something higher than our own world. That is how all Kabbalists started and that is how you will start.
What will happen next? That depends on the effort you’ll make. Ultimately you will attain the degree of Moses, who received the Torah on Mt. Sinai, but when that time comes, in this life or in ten or a hundred lives from now, depends on the quantity and, even more so, on the quality of your efforts.