I feel for you, you have it all on your plate!
You need your own counsellor/ing.
What has he always been like? I mean you were going to relate, for what reasons, is this current behaviour an escalation of what you were already dealing with?
Has this seemed like him not coping ever since the twins came along and now it's just worse, or an illness that has struck out of the blue?
Dont rely on Relate being the consummate professionals. I've heard some pretty awful catastrophes of their counsellors trying to hold abusive relationships in a way that keeps traumatising the victim. Relate are in way over their heads trying to counsel a psychotic patient, and sadly, many dont recognise this.
Its no longer appropriate for you to see the same one. You need your own independent one for you. One counsellor shouldn't be seeing you both separately.
Heay have expressed harmful thoughts toward you or himself.
If he has been aggressive to you in your relationship be very cautious, as they would not be at libeety to share what he has told them and you have been shut out without support.
No wonder you feel angry, frustrated, upset, and fed up.
It important that you only offer what you can in terms of access. I mean, although he's ill, you need to stay well for your dc and not be pushed around and coerced (feel emotionally blackmailed I mean) into complying when its not good timing for you, i.e. needing a rest, middle of meal times, having relaxing time with a friend, or whatever it may be. Your health matters first and foremost because your dc now only have you to depend on. Being strong in this means putting your needs as a priority so you can manage well.
Thinking of you take care of yourself