Please don't be hard on yourself. Psychosis is the meanest and most unbearable situation of life. It is a lonely time where all you want is to side with their reality, for them. Everyone tells them what they think isn't real. And everyone tells you not to side with them. Instead of addressing underlying trauma, councilling sessions are aimed at telling them their thoughts are wrong. It feels like you and your loved one against the whole world.
In emergency, I was told to consider how I would feel if I didn't admit my daughter and we walked away and the worst happened? Realising she had no choice, she admitted herself voluntarily. I know the setting of mental health wards is distressing. I feel your pain, anguish and trauma through every cell of my body, but it will keep them safe for the timebeing.
As soon as you can, please arm you and your partner with trauma councelling. I wish I had done that. I pushed for my daughter through my limited lens of life. I wish I had known she had relapsed. I wish I had been given the right advice by Community Mental Health when she had come off her medications for 2 whole weeks. I wish I had realised how fragile she had again become when she relapsed. I wish I could have been the mother she deserved. After my daughter took her life, I realise wishes just don't come true.
Arm yourself. Learn about relapse. When you tire, reach for help. Trust yourself and walk away from those who don't help. Find supportive people, groups like these forums. Ask questions. Don't let people be dismissive about your concerns. Trust yourself.