So I've slept on it and my thoughts are:
* This is something I should do with someone. That makes it safer and allows us both to articulate our experiences to each other, which I find can give a trip focus and clarity. It ought to be someone I trust, who is also interested in exploring these corners of the mind.
* As Golem points out, it ought to be somewhere secluded, so I'm free to do whatever I want.
* The substance used is less important than the intent.
Ayahuasca would perhaps be preferable. From my one experience with it, it was gentler and friendlier than I was expecting, but that was at a low dose. I understand that it tends to take the user on its own course and you're better off being a passenger rather than trying to direct it. However I'm confident that this is where I need to go next, so I wouldn't be surprised if that's where it took me.
Mescaline has the advantage of being safer. It is certainly a gentle psychedelic, and 15" is a light dose for me.
I'd also consider doing a high dose (8+ tabs) of LSD, but my experience of higher-dose acid is that it becomes very uncomfortable and overly stimulating. On the other hand, this may be perfect for my intentions. On high-dose LSD I will, for example, be gripped with the horror and fragility of existing as an organic body, becoming very aware of the mineral nature of my skull and skeleton, vividly imagining being stabbed in the heart of cut to pieces with a sword, etc. This certainly relates to the fear I'm trying to focus on. Come-up anxiety also tends to be awful at those doses, which again could be an opportunity.
Psychedelic psychotherapy (Mitchi's suggestion) prescribes two methods: psycholitic, which involves taking low doses every one or two weeks over a period, which I'm in a position to do... or, basically, my own idea above of taking a large amount of acid.
As to the nature of the fear, it is irrational. In fact over the last few months, I've systematically destroyed every reason it has to be there at all. But it still remains. I can only assume that, having lived half my life in this state, it's just what my brain is used to, and I've forgotten how to feel any other way. Still, I'm aware it isn't right, as it's constricting every element of my life and personality. Lately I've become aware of it as a physical sensation of writhing panic and dread in my stomach, that seems to arise apropos of nothing.
I'm prepared for it to still be there after the trip. Trips rarely go the way you expect. I've put up with it for eleven years so I can survive another few months.
Before undertaking this, I plan to speak to my therapist.
My main question at this stage is how to trigger the fear. Once I've begun the journey, it could work to repeat a mantra, play music, focus on something that stimulates it or just talk about it. Whatever I end up doing, I really don't want it to hold back. I want to be there with no barriers or defenses whatsoever. I feel I've spent most of my life hiding from this emotion and if I keep doing that, things will never change.