Hi gaz111
Plenty of people with multiple disabilities fill in their own forms and are successfully awarded PIP. I don't think most assessors spend much time considering who completed the form.
There can be red flags for assessors, such as if someone fills in the form with lots of additional information and then claims they have problems forming a complex sentence. Having severe mental health problems won't necessarily prevent someone from completing a form.
I would try not to worry about it.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
