<human>: Condense the text into a few key points

So, I'm on mobile, and I'm sorry for any errors and the wall of text. 

 Like the title says, I never knew my dad. I know that he left my mom when he found out she was pregnant, but that's it. I don't even know his name or how old he was. 

I started therapy about two months ago and brought up with my therapist that I have tried to ask her in the past but she always brushes me off. 
While I want to give her time and try to understand that I may be hard for her to talk about, I also still want to know who he is. 

How can I approach this with her to be able to sit down and talk about this with her? My idea so far was to ask her as a Christmas or birthday present (my birthday is in February) to maybe write me a letter about him and tell me who he is and possibly give me any contact information? I know that it's about 99% unlikely that she has been in contact with him since I was born, but if she could. If not, I would like her...blessing, I guess, to be able to contact and talk to him. 
Thanks for any advice.
<bot>: I don't know anything about my bio dad and would like to know. How do I approach my mom about this after being brushed off before in this subject?