Hello sunsun5 and welcome
I'm so sorry to hear your son is struggling again with suicide ideation, self harm.
We were there back in 2015 -2018 with DD. Extensive, multiple DBT and trauma therapy worked.
The first thing the DBT therapist addressed was to acknowledge her pain. DD told me she realised she did not want to die, she wanted the unbearable pain to go away.
Is this the first time your son has signed up to DBT? My DD's friends and I persuaded DD to give up her job to concentrate soley on DBT and getting well. It worked for her, though it took 3 years for her to be well. Today DD is thriving.
Your post is not dark, it is the reality many parents post and experience. There is no shame here.
Is your son living with you?
WDx