Writing for most takes place in isolation,

There is a good reason for that. Criticism kills creativity, anger stamps on lateral thinking, and the "n" word constrains. Even if the harsh words are being spoken to someone else. And we all want to champion "our" efforts.

I have written in isolation, but two of "my" best songs prove GBS's Golden Rule. One was a song that fooled more than a few into thinking it was traditional, written with three scrapping kids and a nanny at the same table. The mother (girlfriend at the time) saw what I was doing and made sure I was not interrupted. When you dun gotta get the words out - they just dun gotta flow. (Thanks Angela)
The Kinttershanty was written as a collaboration, Lucie was trhe technical expert and mostly watched in amazement. Peter, who I had written with in the past, was doing the one thing that could have killed it - using the "N" word, "no". My reaction was to counter with a, "Just write it down, now what was your idea?". By the time I had knocked the conceptual ideas and tunes into a "Whole" he had great difficulty remembering who suggested what - until I pointed-out a line with four joke ideas in it - he thought it was 2!

So the trick is to not let the negatives interfere - how you do that is not so easy to predict. But recording and moving on is my suggestion. Telling the collaborator "not to" just gets you stuck on one idea. Arguement.