That sounds incredibly tough.
Is the GP aware of how bad things have become?
I don't know if any of this will help, there are probably others better qualified to help and who will hopefully post, but...
This is clearly having a substantial adverse affect on her daily activities. If it has lasted 12 months, or is expected to last that long, then legally she is disabled.
Does she receive PIP? If not, can you support her to apply for it? The disability centre near me runs sessions to help people prepare their applications - check if there is similar near you before you attempt it. There is also a website which currently escapes me that has really clear step by step guidance on how to prepare a claim and ensure you accurately portray the level of need. Have a google for PIP application guidance.
PIP could be used to pay for a support worker to accompany her when she goes out for example.
Might also be worth pursuing a social care assessment from the council to see if she qualifies for support that way.
Also, her mental health is clearly suffering - understandably so - and she might benefit from being referred to the community mental health team (CMHT) for assessment if she's reached a point of becoming socially isolated and too afraid to leave the house. This is far past IAPT (the counselling or CBT you refer yourself to, or that GPs suggest as a first line). That would also secure her additional support.
Mind have an info line you can call for advice on the mental health aspect of this. Details on their website.
Be aware, to help her get access to the care and support she clearly needs you are probably going to have to be more pushy than you're used to being. Putting on a positive front won't help as it just minimises things. You need to make sure professionals are receiving the brutal truth of how bad things are.