Count the last full weeks wages for the last 12 weeks only

Hello, please find attached my worksheet, of which I am very proud.
What I am looking for is in column L, to automatically add the last 12 weeks together where more than 38 hours have been worked. But only the last 12 times. This may not necessarily be the last 12 weeks however, as some weeks may not contain 38 or more hours.
I just want column L to find the last 12 times I worked 38 hours or more from column D, and add the total for those weeks from column C.
At the moment I am having to manually enter a formula every week into column L... Which will change most weeks depending on if I've worked 38 hours or not.
So the formula in L54 would be manually entered as....

=SUM(C53,C52,C51,C47,C44,C40,C39,C38,C32,C30,C27,C22)/12

...to get the average of the last 12 weeks I worked 38 hours or more.
But this changes almost every week... Every year. I've been filling it in manually now for two years....

I need it, as my holiday pay is paid as an average of the last 12 full weeks I worked including overtime, bonuses etc.
Any help you can give is greatly appreciated. In pretty new to excel, so don't know about vba or tables. It's taken me a lot of time and study to do this worksheet, so please try and go gentle with me.
Thank you.
Mark