Fed up with DWP!!!

I filled out the pack to apply for PIP back in September & returned it recorded delivery. I was struggling to get it filled in on time, so i rung them & asked for an extension on the date which they kindly agreed to. I then received a letter saying they will not look at my application because the forms were sent in too late! So i rang them explaining i had sent the form back recorded and they should have received them on time. He confirmed my completed forms were received on the due date, but unfortunately the letters have crossed in the post! He said not to worry & he will sort it. I then received a text saying they will agree to go ahead with the new claim application. So i waited & waited...I finally rang them in November asking what was happening with my application for PIP. They then told me because i was late sending the pack in it will be sometime in the New year! I finally received a letter saying i have to attend an assessment on the 1st Feb. So 5 month after i filled out the form! Yesterday afternoon i received a text saying i had to ring them straight away about my face to face assessment? They have only gone & cancelled it because of staff shortages & now i have to wait another 3 weeks. So another 3 weeks of being anxious & worried I have been diagnosed with Dyspraxia, Fibromyalgia & currently being assessed for high functioning Autism. My question is if successful can i ask them to backdate?
Thank you

Debbie

Thank you for your reply Gordon.

This is a new claim for PIP & no I have never applied for DLA.

So i am hoping i am successful

Debbie