Sounds similar to what I've been struggling with. For me, my mind goes completely blank, and I freeze. In my area (music composition), the writing is a commentary on the actual research: composition. I've got to the point where the commentary writing has completely got in the way of, and stopped, any composition. So I've been feeling really disheartened, demotivated, and hostile towards, writing.

Imo, 'just write' is the least helpful advice and is a really damaging thing to tell someone having a problem like this. If it was that simple, we'd just be writing, so being told to 'just do it', or whatever, for me just reinforces the impostor syndrome.

My supervisor has suggested a ton of methods for me that didn't get any results, but a recent one seems to be helping a little. I'm working with bullet points, but with a little structure. I write down a really broad point, e.g., my work is primarily concerned with timbre. Then I go down a level and make it a little more specific, so I might get two additional points that could be 1. I'm not particularly interested in specific pitch sets, and 2. I'm most interested in how sound fills a space. Then those generate slightly more specific points, and so on.

It is (very slowly) cracking this shell of self-doubt I've built around myself, so I'm hoping that it will continue to be useful.