Hi OP
firstly, huge sympathy for the current state of play. My youngest DD (now 28) was diagnosed with OCD, GAD and depression in her mid teens. She was self harming and had suicidal ideation. We accessed help from a child & adolescent psychiatrist at a local Priory and she did ‘recover’ and manages a regular life (uni, partner, job, mortgage, dog) nowadays but has to tightly manage her life!
for her, the OCD was the hardest thing, as other posters have said, it can’t be cured but it can be controlled. She had very successful art therapy, OCD focussed CBT and SSRIs for the depression. The OCD gets more heightened during times of stress, still.
as a parent, the hardest bit was to constantly be trying to work out if there was a crisis that she needed support in managing, or if it was a situation that she needed to work on herself. The compulsions were usually when we were advised to step in (back door being locked at night was a specific one, I’d let her go up and down the stairs checking up
to 15 times and then I would step in ‘darling of course the back door is locked, I will check as you know I wouldn’t leave it open over night’ as we’d agreed this) but there was a significant amount of time where the emotional weather, for want of a better word, in the household was controlled by her. We had also been really let down by CAMHS and it was hellish getting her to the Priory to ‘see another person who won’t believe me’ but as she was 14, we managed. With your adult DD it might be harder. Is there another adult who she is close to and trusts who could talk to her about diagnosis and treatment?
you know that this is highly unlikely to be about your partner, this is her current fixation and explanation and it’s likely that it you did break up the focus would move to something/someone else. You also need support and a place to offload as living with someone going through this is a hellscape. does school have any pastoral care that could be offered to younger DD? My elder DD found it useful to have someone outside the family unit that she could really vent to about her frustration with younger DDs diagnosis and behaviors!
as an aside younger DD has been diagnosed with AuADHD in adulthood, I think there can be some co-morbidity of these conditions.
I’m sending you all strength, it’s very difficult for you all.