pip invitation letter

good morning all
well yesterday I received the dreaded brown envelope and my heart sank, I have been on dla indefinite middle rate care for a number of years. however I now suffer from multiple problems that were not there when I made original dla claim.
originally I suffered from severe depression which I still have and am heavily medicated for, but I now have arthritis of the spine, knees and hands as well as fibromyalgia and thyroid disease.
I am worried sick about applying for pip as I have read the horror stories on here and with the above conditions I feel it will be an uphill struggle to fill in the forms.
I have a supportive doctor who will offer what limited support they can and have been given household aids from the occupational therapy department to try to help at home.
can any of the mods offer advice on which of your guides I should be looking at to help me fill out the forms.
sorry for the long post and thankyou in advance for your reply.