It sounds to me like you really, really need your mom but your mom is dealing with her own trauma so she’s not available to you. It may be that your therapist is responding to her distress and maybe isn’t seeing yours. So here you are, wanting help, but not getting it from the two people who would usually be the people to give it.
Instead of sending out “hints” about the level of your distress, like having scratches on your arm, you need to be more direct. Show your letter and this response to your therapist. I hope the therapist can help you and your mother be more supportive of each other. You need a mom. She is in such pain that she probably doesn’t understand that she stands to lose the relationship with you as well as the relationship with your dad. You need help expressing your anger and sadness without being self-destructive. If the therapist can’t treat the two of you, ask for a referral for your own therapist.
I’m very glad you wrote. Now please take the next step and have an honest conversation with the therapist.
I wish you well.