Hey Zoe, I know where you are. I have been there too.
How do you write? Are you a pantser or a planner? I love pantsing and seeing where a story takes me, and I'm all for the organic creativity, but sometimes I do have to step back and do a little planning. I often find it does calm me, because when I get stuck, I go back to my plan. I also write some character bibles... a little bit of background. These also help for plumping up backstory subtly, and for helping me know who my characters are - what decisions they've made because of something that happened in their past - or didn't!
I don't know if I'm helping, or just sounding off. The above techniques have worked for me. Plus, the other thing that also works for me is (dare I even say it) word count deadlines for the day. This forces me through my indecision, and I just have to get the words on the page. I might delete a load later (the next day), but at least they are there - and sometimes false starts are good for seeing where a story shouldn't go. Even Donna Tartt takes missteps, I saw an interview where she said she'd gone on an 8 month detour on "The Goldfinch". She deleted 8 months of work and started again when she realised it wasn't working.
I think what also calms me is that I have actually messed up on a whole book before. I rewrote it several times, and being stuck on one book took many, many good writing years of my life, however, I'm still writing and loving it and being frustrated with it and getting better (I hope) at it. I've got a very different, but more marketable book than the one I set out to write, and I learnt a lot along the way. I suppose I'm trying to say that nothing is ever lost, and that in the pitch darkness of the struggle, we can't always appreciate that "this too shall pass".
As for the choral music, and the beam of light (ha ha), I don't get that ever. There is sometimes a little frisson of "certainty" I get when I've written a good scene - the downside to that is when it's a scene an editor or beta recommends you cut. Oh, killing your darlings.
Oh, and I adore your dog! I think he/she is one of those with a purple tongue?