I now feel its time for me to walk away. He's clearly panicked about me abandoning him but is still treating me with such contempt. He also won't admit this behaviour is not normal -therefore I cannot help him.
Once you've decided to leave, and you've clearly communicated that's the case, there are a few approaches to keep in mind - for your benefit, and for his...
I suppose I'm asking how I make the break now. We don't live together have any kids together or shared finances so it should be simpler but I worry about him either becoming volatile or maybe even hurting himself. I don't know what to do.
We often discuss FOG - Fear, Obligation, and Guilt - as a framework to describe the dynamics that can keep many of us in unhealthy or even abusive relationships. Your concern about volatility (not good for either of you) and self harm (not good for him) seems like it's right out of the FOG scenario... you can read more about it here:
https://bpdfamily.com/message_board/index.php?topic=82926.0 Even if you don't relate to this particular framework, I would encourage you to continue to focus on what's right for you. i.e., shift the frame from prioritizing him to prioritizing you. You're already on the path. Keep going.
e.g., when you fully understand and accept that the relationship you thought you had, or hoped to have, cannot happen - you'll realize that letting go of him is actually taking a step toward a chance to be healthier in the future - for both of you.
I often post this: in my journey, I came to understand that "radical acceptance" didn't mean I had to accept my disordered partner and her rages, perceived insults, cycles of depression and anxiety (frequently attributed to me) - rather, the thing I had to accept was what I could change in myself. This realization was a gift.
I am planning some therapy to get over this whole nightmare as i'm completely drained mentally and emotionally. This man has been in my life for 24 years and i've been in a relationship for five, its so hard to walk away from someone you love so much but who is destroying you...
Agree, it's hard. Good step in the right direction to start therapy. To answer your original question re: "how to do it" - Perhaps the best advice I can give you at this point is: Try to disengage by avoiding interaction. If you must communicate, keep it short and to the point - don't take the bait to get lured into an emotional confrontation of any sort. Be neutral, flat, and boring. i.e., don't feed the fire - for him or you.
You're at the point where you know what to expect - and hope to avoid it. So, it's up to you to take full responsibility for what you bring to the table. If you want to avoid conflict, then avoid conflict. In some ways, it's really that simple. That means: Don't engage. Give yourself the chance to fade out of his life - and let him fade out of yours.
Does this make sense?
Hang in there, and let us know how you're doing.