I've previously been a landlord to a few properties and I would be furious at your giving mental health reasons as an excuse. There is never an excuse to get out of paying what you owe and I don't want to know about people's personal problems when it comes to financial agreements.
I work with clients with mental health problems, doing mainly welfare rights work but also some basic debt work and coaching in budgeting.
The challenges many people with MH issues face in dealing with what, to most of us, is straightforward stuff, are often insurmountable and support services that help them deal with it have been cut to the absolute bone. Poor mental health is not an excuse, it's a reason, @Attelina.
OP, if you have a CPN or any other sort of support, ask for their help. They may be able to refer you to a service like the one I work for that can help you with this. If not, get an appointment with the CAB. If you're on UC, put a note on your journal asking the DWP to set up an "alternative payment arrangement" (APA) so that your rent goes directly to the landlord; if on HB, ring the council and ask them to arrange for your HB to be paid direct. While that's going on, work out how much you can afford to pay on top of your rent to reduce the arrears. Then speak to your landlord and explain how you propose to clear them.
If you have a trusted friend or family member who can support you to do this, ask for their help. And for the next few weeks, don't buy anything that isn't essential.
I wish you all the best in dealing with this, and really feel for you.