Hi So Leigheas,
I am sorry that you

are experiencing this.
I too deal with SI and it can be totally disabling. I can be feeling great, having a good time or just going about my normal day and out of nowhere bam! a rush of thoughts of how to do it and that I should do it and sometimes I can see myself doing it, playing the whole thing out but seldom no reason as to why. I shared this with my therapist and a pdoc that I use to see and they could not tell me why I think/feel this way other than perhaps a chemical imbalance. My therapist said it's probably also related to PTSD, cause I feel guilty for surviving a major medical trauma that 90% of others that it has happened to die from.
When these thoughts overwhelm and consume me, I tell myself its my thought process but not what's in my heart and I distract/redirect my thoughts/activity. I tend to write out how I am feeling in poetry and may post it in Creative Corner forum and it has helped. I've read most of your poetry in that forum, you are a very talented writer and you clearly convey how you are feeling. I encourage you to keep writing especially if it helps.
sending well wishes and hugs your way