You aren't alone in this, just yesterday I jerked kinda funny to prevent something from falling in the tub and thought that I might have hit my head, even though I didn't feel anything, and sat in the tub re-enacting to convince myself that I hadn't done anything. That's the first time I've done something like that without actually hitting my head, but when I have hit my head I've re-enacted how fast I was moving to get an idea of the speed or forces involved.

I try to get through moments like this by recognizing my anxiety for what it is, and anxious overreaction to a small stimulus, and reassure myself that if any damage was actually done, it's only to the muscles/skull. Also, and this is just my experience, but any of the headaches related to these small headbumps have eventually either totally gone away or subsided significantly, so I tell myself that even if I did some kind of damage, the symptoms are likely to be short-lived. Finally, I try to tell myself that even if I did get brain damage (as exceedingly unlikely as that is), there's nothing I can do about it now, and getting anxious only makes it worse. It's easier to tell yourself this than actually believe/enact it, but I think simply telling yourself is the first step.

I've also been on lexapro for about a month, and it seems the visceralness of anxiety has improved, but I still have some anxious thought patterns.