Running, we have 'spoken' before on another thread.
My thoughts are - have you been able to access any counselling or systematic support through CBT or other approaches for unpacking what happened last time? Maybe treating it as a traumatic incident and giving it the attention it needs would help? I'm sure the birth trauma association can point you in the right direction, failing which there is the BACP where a postcode search can help you find someone? Perhaps one debrief alone won't do it, but perhaps a set of five long sessions will help in some way? Tokophobia is very real, and you need support to help you through it.
Secondly, your DC2 Does not know what position DC1 lay in. It is a different sperm that met a different egg, in a changed body, at a different time, to grow into a little body of a different shape that will lie in it's own unique lie. All these point towards high chances of a very different birth.
Third, if finances permit, you can always get a growth scan at 30 weeks, or ask midwife to palpate And tell you at that time which way baby is lying. I have done so. Baby had a tendency to be posterior. Daily exercises from Spinning Babies, completely altering my resting positions and now as space reduces baby has come to lie on my left, head down, and sideways facing, back to belly. I'll be trying to keep things going that way but sometimes people have pelvic shapes that get in the way. Again, a private presentation scan at 35 weeks will give you some info on position as might an experienced palpating of the belly. I say this only because you e concerned about posterior positions.
For those who are doing hypnobirthing though, I'd like to add - there are many models of hypnobirthing ranging from ones that totally denounce medical intervention and ones that aim to support whatever happens on birthing day. Choose wisely. I'm expecting my first next month and I have the strong feeling that I know far more about Keiland Forceps and have planned more about the possible EMCS than I know about hypnosis. This is because I entered oregnancy with tokophobia and researched all of this before I heard of hypnobirthing. That's a good thing, I think.
So, choose wisely. I deliberately didn't choose the Mongan method. I chose another programme, a different teacher who was a retired nhs midwife. She talked freely, openly and non judgementally about all her various births - ones in car parks, ones in a dark pool, ones with epidurals, forceps, EMCS. And she taught me scripts to make the most of any of those situations. The tape I listen to everyday has visualisations of different paths to the sane destination, and the script says about this alternative path how it is good to remain open. My hypnosis tapes specifically talk about "whatever turn my birthing takes" and "whatever happens on birthing day". So, as I practise relaxing, and breathing and yes, have planned a pool hypnobirth, I am keeping a careful eye on my baby's position, have toured and asked to see the forceps, ventouse, epidurals, and I believe my hypnobirthing teacher when she says that she cannot promise a pain free birth, but, that my practice will be useful to face whatever turn my birth takes. So there are many classes and many approaches - Mongan wouldn't have suited me for the very reasons Running talks about. So I've lifted snippets from Mongan to blend with my own practice and prep and of course while I have been researching diamorphine and forceps.
Birth next month. Who knows what turn it will take? Pool and light moaning? Scenario A? Or 60 hours of sawing pain and an EMCS? Scenario Z? The best I can do I think is to rest and sleep well, know how to relax and breathe whatever happens on the day. So if you're doing hypnobirthing it's wise I think to do it with a mind as open as the beautiful open seas in the relaxation tapes.
Running, I feel treating it as traumatic stress and giving this the support it deserves will go a long long way yo help you, in a way an Internet forum cannot. You didn't fail. There are many routes to the same goal.