The thought of you catching the bus might scare your mother, which could be why she reacts the way she does.
If you want help from her, I think your best option is to give her the facts. Tell her that you are not messing about, you are not trying to put pressure on her, but that you need help, you are not getting that help, and that suicide is now a very real possibility. You should also tell her that the kinds of intervention that have been tried in the past do not work for you, and there is no point repeating them.
It would help her, I think, if you could describe the kinds of intervention that you think would (or at least might) help you. You mother is not a professional therapist, and she may have no idea how to help you.
Of course, you must be prepared for the possibility that you won't get any real help from her, either because she can't or she won't help. You should think how you would deal with that, if it were to happen, before you talk to her.