Hi Shelby
I'm sorry that this has happened to you. You first want to ring the DWP and ask for a copy of the assessment report (if you haven't already). This is a PA4, and it will give you greater details of the assessor's thoughts and what was behind the decision.
You have one month from the date on the decision letter to put in for a Mandatory Reconsideration (MR). You can learn about the process from our Guide to PIP Appeals (which includes the MR information)
benefitsandwork.co.uk/guides-for-claimants/pip.
The assessor is just giving their opinion, and it will be your job to go through the report and point out where you think they have got it wrong, highlighting the evidence you have already submitted. Don't fall into the trap of calling the assessor a liar (as tempting as it might be)- use words like "they are mistaken...", "they have misunderstood...", and "they have ignored..."
BIS