The startles are like an electric shock or a punch in the gut (complete with a grunt!), so it's hard to work through them.
Your description sounds pretty accurate. Sometimes I have been surprised by its presence.
The fact that it is a "response" might be telling. Trying to immediately locate the trigger and what you might be responding to can be a little challenging, however.
After the shock, I just describe to myself what happened (bumped my foot, heard a loud noise, etc.) and then tell myself that the startle will fade away.
Reassuring yourself at that point seems like a good idea.
Although in some way this might seem like a "body memory", the body can't really remember without the help of the brain. Allowing the experience to come through the body safely is a lot of what traumatic reprocessing is about.
Although it seems like it would be good if this lessened the shock a little too, I'm actually not sure that's such a good thing. The shock after all is a honest and natural registering of something that happened in our environment.
Of course directly taking away the hypervigilance is not really possible. The work can seem and endless and it can feel hopeless when the shocks keep reappearing. But as awareness slowly increases, so does emotional intelligence because the body and mind allow for a little more information each time you take in a slightly bigger picture.
Hope that you can not only find some peace as well, but appreciate it each time you have it.