Just wanted to add that I do know how you feel.
In my case it was my dad, when I was 17 I begged him to leave our house as my parents marriage was at crisis point and my life was intolerable. My dad refused to leave or change, and timed an overdose for when I came home from college alone. He, too, had me begging and pleading with him while he continued to take the pills in front of me. It was very traumatic and he ran off to the local park to continue and he was eventually sectioned, I dealt with the emergency services alone.
I understand now that having me witness the overdose installed a button in me, which he then continued to 'press' whenever it suited him.
If he wanted me to do something, or not do something (or just to spoil or disrupt something important), he would start with the suicide threats and I'd comply straight away (I understand now that I had undiagnosed PTSD).
I have had to go NC this year because my children were starting to become programmed this way too, and that was the last straw. For years they have seen my dad say Jump! and me say, How high?
Earlier this year, he threatened suicide in front of my 11 year old as a means to coerce me into doing something for him and it was then that I'd had enough.
Sounds awful but I'm glad your DP doesn't have a relationship with his DC as he would do this to them too, and it really would ruin their lives.
Also, is there any other suicide history in his family? I only ask because I had a hunch that my paternal GF may have died from suicide (only because his cause of death was always kept secret by my dad and his siblings). A few weeks ago I went on FreeBMD (births marriages and deaths) and did some family research, then I sent off for my GF's death certificate. Sure enough, he died from acute barbiturate overdose.
I can't help feeling that my dad saw the devastation that his father's suicide caused the family and so he decided in turn to inflict that on me, his only daughter, at 17 years old by making me witness his. It was an obvious tool for him to reach for. He is a very unwell person but I have had to end the cycle.
Best wishes to you, please detach from him and do not feel guilt.