Immortal and the Madman (The Immortal Chronicles Book 3) by Gene Doucette

I go mad every now and again.
    That’s probably an outmoded way of describing what I’m talking about.  I appreciate that there are truly insane people in the world who are suffering greatly because of some version of insanity, and I have never been that kind of mad.
    I will explain.
    Nowadays there’s a thing called psychoanalysis, and whole sections of the medical world devoted to understanding and treating insanity, and it’s all done rather respectfully and humanely, and that’s fantastic.  But for most of history we basically had functionally crazy or batshit insane and that was about all.  There was no shortage of possible causes, from demonic possession—a personal favorite—to a huge range of supposed mental deficiencies that could be summarized as “being an outspoken woman.”
    There were different kinds of crazy is my larger point.  Some kinds of madness resulted in the sufferer being unable to function within society at all, and other kinds were less obvious, and could even be described as a species of uncontrollable creativity. 
    Losing control of my mind is a real problem, because I already live in a reality that’s difficult to grasp for humans with ordinary lifespans.  (That sentence by itself, written today while I am in full command of my faculties, could be enough to get me committed in the right circles.)  Having lived a life that sounds like the rants of a lunatic is a real problem if I start to actually rant is what I’m saying.  Losing my grip on things puts me in a great deal of danger.
    The problem is, lucidity can be an act of will, and sometimes I lack that will.
    I’ve always assumed that aside from a few quirks—I don’t age, I don’t get sick—I’m basically as much of an ordinary human as there is, and if that’s true I’m working with the same kind of brain as everyone else.  But since I don’t die (or rather, since I haven’t yet) that brain is sometimes working very hard to just keep everything straight.
    To give a simple example, let’s talk about pattern recognition.  Seeing something and recognizing it as something you’ve seen before is a survival skill.  For instance: I see a leopard.  Last week, I saw a different leopard kill my friend, so even though this leopard is not the same one, I know not to pet it.
    That the mind can compare a new object or animal or person to one from prior experience is a good thing.  It jump-starts the fight-or-flight instinct, and it works exceptionally well for people who are drawing from ten or twenty years of experience on the planet.  But then there’s me, with the same kind of brain and the same kind of instincts, only I’m pulling from sixty thousand years of information.
    If I’m not careful, I’ll overreact to things that aren’t important and miss something I should be paying attention to.  I basically have to be on very good terms with that part of my brain, all the time.
    There are times when I lose the capacity to do that.
    *   *   *
    I usually recognize when it’s about to happen, and thank goodness.  I have to self-monitor, because I don’t often surround myself with people I can trust with my life and also with my secrets, so if I start to babble I’m generally on my own.
    The first indication that something is amiss is when my heart starts racing, more or less randomly.  It’s what the body does in response to an apparent threat, and it’s great when there actually is one, but when you’re in a situation without any apparent danger it’s scary as hell.  As my heart rate increases and I start to sweat, I begin looking around for things that I can use as weapons and for weapons that are about to be used on me, in case I’m wrong about the danger thing and my instinct is accurate.  Sometimes it is, and this is just worse because it means I can’t ignore every instance. 
    Here’s an example.  I was in this tavern one evening in what I think was probably the mid fifteen-hundreds.  It