Matted collie

With my old boy I just do a little bit each day just to keep on top of things as he is very fluffy, arthritic, lumpy and not comfortable when groomed. Just do a small bit at a time, whatever she is comfortable with, maybe use a small pair of round end scissors to cut down the length of a matt to make combing out easier if clippers stress her?
There is a groomer on here that may have more advice.

 

Have you thought about asking your vets to sedate her and clip her coat right off and start again ...at least she will feel comfortable and you could then keep her coat shorter ...i have a lovely rough coat lurcher who i clip right back in the summer as she hates being groomed and gets hot

 

Havev a look at Furminators. I've found the cat version good for my long-haired cat who hated being groomed - he's much more tolerant in general now.