Hi
Belle78 Sorry to hear about all the troubles you've been going through.
From my experience, the DWP won't be charging you with fraud at this interview, assuming it's the first Customer Compliance interview you've had. The absolute worst that could happen is that they decide to call you back to do that at a subsequent interview.
I suggest you try to get some proper advice before you go to the interview, either from the CAB, or through your community mental health team. I know that in my area (Haringey in London) there are special benefit advice sessions available for people with mental health issues.
From what you say, I would hope that if you have not given them info you should have, you will have a reasonable case to suggest that you didn't do it deliberately, but with all the other stuff in your life you accidently let it slip. But please don't take my word for it, get some proper advice.
And also, try to get someone, friend, member of family, support worker to come to the interview with you, even if only to give moral support and act as a witness in case that's helpful.