Is there any way for someone to practice it, and become more agentic with time?
I have not sought counselling about it, but two mind hacks have been helpful to me.
First, are the notions of the inner child and inner adult. The idea is that when you are feeling passive and powerless and indecisive, you are reverting to how you felt as a child. Making a conscious effort to imagine your situation from an adult perspective (and even contrasting in your mind how your adult persona and your child persona view the situation at hand) can be very helpful.
The second hack is to imagine that a good friend is asking for your help in dealing with his or her own sense that they lack agency. What would you say to that friend? The power of this technique is that we all tend to rise to the occasion when called upon to help a friend. We will want to convey confidence that our friend can indeed get the better of this debilitating attitude.