NYE I crossposted with you.
do you have to prove yourself all the time, as though your default setting is that nobody will believe you?
Yes! I've been there, I was there for a long time. My dad gaslighted the hell out of me (he also did it to my mum during his numerous affairs). Being believed is incredibly important to me. If I ever feel that someone doesn't believe me (about anything) I can actually get quite aggressive over it, even now.
Trigger warning
Regarding your questions about the physical pain of birth bringing back memories of abuse. I can't answer this from my own perspective as I haven't given birth and my dad didn't penetrate me. However the sensations you describe would be consistent, in my (non-medical) mind, with penetration of a child. At that age you would be unable to distinguish between sites of pain and would experience anything in that area as "my tummy", I would think.
I have read an account of a male survivor who was anally raped at around age 7 or 8 and described the pain as being in his stomach.
I can recommend Alice Miller's book "The Body Never Lies" which recounts the effects on the adult body of the abuse suffered as a child (physical as well as sexual.)
"Perhaps I just dreamt he threatened to kill me if I told anyone"
Unlikely - it's a very common threat used by abusers to children, especially young children.
"The cruel part is that memories aren't clear, especially as they were buried for so many years. Why are some parts undeniably clear while other parts muddled? It causes doubt and confusion and is cruel. "
Yes, this is very common. Current thinking among mental health professionals is that it is caused by the way the brain stores memory. Normal everyday memories are laid down in a part of your brain that deals with memories. It's like a filing cabinet - the memories are stored in their place and if you want to remember something, your brain goes and "looks it up".
However, with traumatic events, because they are so terrifying, the brain is not working to "file" the memories in the normal place, because it is so scared and doing everything possible to just survive this experience. So the memories end up scattered all over different parts of your brain. You may have difficulty recalling them on demand or being able to remember "what happened after that?" You may also suddenly come across a memory because you are reminded (triggered) by something familiar to the memory. (I think this is what happened to you when you gave birth.) And once the memory comes to you, it's hard to then banish it again. It doesn't feel like a memory, it feels like you're right back there, experiencing it again.
I hope the above makes sense. This is how a psychiatrist explained it to me. There is a new technique that some therapists are using called EFT or EMDR which is designed to "re-catalogue" traumatic memories in their proper place.