You didn't bite him, because by not doing so you were increasing your odds of survival, of making it out of the situation relatively in one piece. Your body was just trying to protect you as best it could.
You can't control arousal. It's a natural bodily response to certain stimuli, even if it makes you feel guilty - it's not your fault. It was unwanted. You didn't choose to let that stuff happen. You had no choice in the matter, it was forced on you, and your body reacted in a very common way - a way that probably saves many people's lives, or saves them from further injury.
I myself think "why didn't I bite his dick off, instead of fighting the urge?" - I feel guilty over it. I feel disgusted. It makes me feel like I -chose- to let it happen. But, I did not choose. There was no real choice.
If I bit his dick off, or tried, I'd probably be dead right now. Or I'd have sustained more broken bones. Yet that feeling of disgust at myself, guilt, shame - it's still so hard to try to shake off, or shove away. I have to work on a lot of memories to make that go away, I think. But it helps me to think of this: my body was just trying to survive it - and I did.
Every time I did fight back - I was injured. Every single time that I can think of.
Sometimes fighting back is the only option for survival - sometimes fighting back is the worst option for survival. Very often in sexual assault, fighting back only makes the outcome worse - more injuring. At least, that is my experience. Every situation is unique - but a common theme with a lot of sexual assault trauma, is feeling bad over having it happen, feeling guilty for it, feeling like you could have fought back or done something and that it's your fault it happened - that your body's natural reaction to things is your fault.
But it isn't your fault - you didn't have a choice.
What happened happened, because your body was just trying to stay alive and in one piece. To not escalate things - to not fight back - because that can be very dangerous - and here you are, alive and able to post.
I’m struggling with a flashback/scene/memory from which I am very detached. I feel no emotion just a sense of unbelief whilst watching.
What you describe sounds a lot like depersonalization/derealization
It accompanied many of my sexual assaults, along with many other traumatic incidents, and if I weren't constantly taking naltrexone, I'd be constantly depersonalizing and derealizing.
I think my brain relied on it too much during the trauma, and so it became my brain's go-to defense mechanism, which, when the trauma is over and you're trying to live a normal life, is very handicapping. It makes it very hard to get through life, and to me derealization and depersonalization feels scary, makes me freak out internally while my body does things of its own accord, and everything feels unreal, and it's like I'm watching a movie of myself.
Are you in therapy or anything like that?