Don't hesitate to write a letter you don't send.
When I was younger and my parents' alive, I used to call them after I left home and had the kind of experience you describe. (& I wrote letters they didn't answer...) It went on for years (and my husband at the time would say "Why do you do that?"---he pointed out that I called my parents when I was depressed and came off the phone much more depressed)
Jump ahead and I end up being the one to help my parents, --- through my brother's death, my dad's cancer, my mom's dementia....life is strange...(I was the "incompetent" one in their eyes-------------oddly, at work I was the 'competent' one...) I did learn that much of their behavior had nothing to do with me....
This is NOT a recommendation that you keep trying for something you cannot get from your mom. Just that you realize that your mom isn't able to give you what you need and that it is not about you. And that is what hurts us---it isn't about us.
It is time to get busy in other activities. Work with your T on coping skills that do NOT involve calling your mom. Now, I am simply grateful that my own kids are adult, productive, and know I love them and they can call me if needed (they rarely need anything)---and that I have open invitations to visit them...so, something went right after all....