Hey OP it sounds like you've been through a lot. And all of it seems to have had you severely impacted, but simultaneously as a "secondary" person (if that) in the underlying situation. What I mean is that your DH had the psychosis, but you were the one who actually suffered. As a kid your mother has whatever issues, but you had to walk on eggshells.
Have you seen movies where there's a scene with lots of characters are arguing, or kids going crazy making lots of noise and the central character stands in the middle of the chaos, becoming totally overwhelmed until they cover their ears and scream at everybody to be quiet/stop? Your self harm reaction seem to me like that. Things around you build up and it gets understandably overwhelming and you do the equivalent of covering your ears and screaming, only you hurt yourself instead.
It actually makes quite some sense.
Your DP went waaaaaay over your boundaries repeatedly and your mother most recently did it too. It's extremely stressful for you (those experiences would be for most people) and so you have an extreme stress response. However, while there's definitely logic in there, it's not helping you in the best way, because you end up damaged. You don't deserve to be hurt, for any reason, physically or emotionally.
I'm confused that your therapist didn't have much to say though. Is your therapy to deal with things related to this? What sort of qualifications does your therapist have?
If you're not being helped with this in any way, then in the interim you can try some things yourself (slowly, baby steps with zero pressure to "do it right" or the like). A PP mentioned some things that help with stress reduction. You can give some of those a try. You also feel the need to hurt yourself though so perhaps there's a less damaging way? Some people find holding an ice cube against their skin can be useful. Others use an elastic band on their wrist that they can ping. These things are all best practiced when you're not in the thick of it, so when the next occasion of overwhelm occurs, your brain has already got different pathways to different options to choose from.
Maybe some of that use useful, maybe none. Do what you feel you want to do. You are who is important here.
You can also ask your therapist directly to help you deal with this. You're used to being number 2 and not being listened to or prioritised, so I can imagine it's hard to ask for something when you feel unheard, but the therapist is only there to help you. It could be that she feels you're working on something else and whatever that is will help you with this. It could be that she wants to focus on what you're doing now before moving to something else. She may not have realised it's important to you to work in it, or that her lack of response was confusing for you. Or many other things. You're allowed to ask her though. She won't think badly of you for asking and she won't be angry at you.
I have gone through extreme behaviours that hurt me due to overwhelm from the behaviour of my STBX. I was also seen as a secondary/tertiary/lower down person to everybody else with my boundaries completely trampled on over and over again. So I recognise this from your post. However, I want to qualify everything I've said with that. I'm seeing it through the lens of my experience. I'm not a professional. None of what I say may be relevant or useful. But on the off chance that it is, I wanted to share my thoughts and let you know that I found a way of the behaviours, so it is possible. Good luck OP, you really have my sympathies, you've been through an awful lot.