Okay. Well, I'm not an attorney. But contacting one, or at least doing your own research on the internet, would be very helpful to understand how the laws concerning child custody in your State work.
I assume you're in the USA, and I believe the majority of States in the USA have very similar laws about child custody. You're 15 - at that age the courts seriously consider the wishes of the child with regard with whom the child wants to live and the amount and type of visitation with the other parent. Because they know, at 15, one can be very stubborn and simply refuse to not do things, in general. And, because, in most States, at the age of 16 one can become emancipated from both parents.
I think you might be fearful of telling your mom about how deep your depression is and your SI because you know she cares about you, hearing the news will upset her, and you don't want to upset her. That's very, very understandable. However, her natural instincts and her role as your mother (if she's a good parent) will put your well being and your care far above any discomfort that she feels. Good parents want to know what's going on with their children: the "good" and the "bad" so they can share in celebrating the good and help address issues that contribute to the bad . . . to ensure that the child grows up to be an adult that's as healthy and happy as possible. From what you've written, I think you want to tell your mom what's going on but you just don't know how.
You mentioned that your father and step-mom abused you. When abuse happens people generally have feelings of hopelessness and helplessness. Continuing to be with your father and step-mom, when you don't want to, may be keeping you stuck in feelings of hopelessness/helplessness. What helps people move beyond those feelings is getting in touch with their strength(s) - their power.
How can you exercise your power and change things? Learning about custody laws in your state and taking action. I want to suggest that you write your mom a letter, not an email, but a letter that talks about (1) your feelings and fears including your fear about her becoming upset at hearing the news of your SI, (2) address the conundrum that because of the past abuse being made to be with your father and step-mother for extended periods of time, without your choice in the matter, makes your fears, your feelings of hopelessness/helplessness worse, (3) expresses your appreciation for her getting you to therapy and asks for her continued help in moving beyond the dilemma. Also write out a contract between the two of you that states you won't act on your SI. Then sit down with your mom and read her the letter. (If you want, you can post the letter on this forum for feedback before you sit down with her)
The fact that your mom has sole custody puts her in a powerful position; it gives her more responsibility with regard to your care. And, in most states, if the visitation agreement was made more than two years ago, the courts will take a current look at the situation and impose a different alternative than what's in place now.
You have the strength and the power to change your situation, Minty. You might not feel it right now, but it's there. Dig deep. Start getting in touch with your power and take action steps that let other people know you've got it! Keep posting in this forum and folks here will support you!!!!
:hug:
Drew