@Maisydaisy56
Don’t know how long they were together but for your relationship to have any future you need to sit down & talk through the realities of him going NC with her. Even when she threatens self harm, acts out, makes demands, causes chaos. You need to go through each senario & where he would ordinarily feel compelled to step in & (rescue or save her) get him to play out what would actually happen if he didn’t. Who would respond? What would it mean if he left her to it? What would he feel if she did harm herself? How much guilt or emotion would he feel or responsibility if he did not intervene & stuck to boundaries??
Play it out & get him to be the one to come up with who would intervene etc not you. Test how able he would be able to stick to that. Ie faith in the knowledge that she is not his responsibility. Willingness on his part to try. If anything bad happens to her that’s not his fault. If she kicks off & causes damage somewhere he is not responsible for calming her down etc. ie the police, crisis mh team, family are.
How able will he be to resist her demands? Calls? What would he do if he sees her number calling? Unless no DC he should block. Which family member can he also cut off as they may have passed the buck of her onto him as his responsibility. He needs to hand her back telling them she is vulnerable but he can no longer be called upon to help.
He sounds caught up in a cycle of being manipulated by her (that’s not say she doesn’t suffer from real mh issues).
Or he may be caught trying to save or fix her but got burned out & is doing it out of a mix of obligation, habit & guilt fear that he has no alternative but to.
If he’s truly serious about you & your future he needs to properly take on board how much he’s got to stop responding, and take accountability for how much he’s actually enabling her behaviour. If his guilt/responsibility/fear/care for her won’t let him - either seek help together for him to work on putting in boundaries & sticking to them - regardless of what could be very unpleasant circumstances, or try to take a step back & look at the situation completely objectively & work out if he is actually capable or if he’s too enmeshed to do so fully. If so then walk away because this will be your future.
You say you agree Now that she does have mh issues. Does that mean you initially dismissed as you’ve never witnessed before & didn’t realise how serious it was?
Does the mh aspect him make him feel he can’t fully walk away (as it will obvs exert an emotional pull of pity, concern, fear etc) re leaving someone potentially v vulnerable or unwell??
You may need to get used to her potentially continuing to harass, sabotage your relationship, seek punishment /revenge on you/him (depending on type of & severity of her issues - not saying all people with mh problens do this at all - just that her behaviour seems quite extreme). Is it different if/when she takes medication? Is there a pattern of her “needing” him more at some times more than others? Does she have any stable periods etc etc you need to know from him.
Given self harm/suicidal history of over 20 years I would expect her to have a formal mh diagnosis? Might be helpful to google it so you can get a handle or try & make sense of what you’re dealing with.
Just so can handle more objectively rather than emotively caught up in her chaos. She needs professional & family help. Every time your DP steps in he’s preventing her from getting it. And enabling her to occupy that place in his & your lives.
Good luck.