I think it depends on how you've talked in therapy so far. It's one thing if you just talked generally about your life and issues and your therapist thought that your relationship with your mother was priority; than I think you need to be very specific about what it is you need from the sessions at this point in time. It's another story if you have repeatedly told your therapist that at this point in time you need to talk about your father, and she still insists on another topic. Then yes, if you really don't feel understood and want to go for another therapist, that may just be what you need.

I think you could explain to your therapist what you've said to us here - that you understand the importance of your relationship with your mother, but your need is to talk about your father in therapy. Perhaps ask if she is aware that she seems to be pushing another topic and if so, why she does this. She may have good reason or she may just not know what you need. I think communication is key.