What can I do about severe vertigo and showering? What would cause severe vertigo from a hot shower? Are there any ways to decrease the severity of the vertigo, other than taking cold showers?

Depends on cause. Vertigo is defined as a perception of movement of you or your surroundings often accompanied by nausea and vomiting. If lightheadedness then you feel like you want to pass out. If lightheaded then i would be concerned about heat making blood vessels dilate lowering blood pressure which may indicate autonomic dysfunction. If it happens with eyes closed only then that may indicate weak inner ears.
Vertigo . Vertigo is a word that is often confused. Many people use it interchangeably with "dizziness" so it is important to know what you mean. Sometimes people mean getting light-headed, like when you stand up too fast and feel faint. Other times people mean they have trouble keeping their balance because their coordination is disrupted. Both of those examples are not vertigo. True vertigo is the sensation that your body is spinning. Anyone can give themselves vertigo by spinning around very fast and then stopping, like on a playground toy. Under normal circumstances, the "balance" signal comes from the semicircular canals, through the vestibular nerve, and into the brainstem and cerebellum. It is your body's way of knowing if you are spinning so you can compensate. But some people get vertigo for no apparent reason. This happens when some part of that circuit is disrupted. From this question, it sounds like you have true vertigo. Neurologists have a test in which they put hot or cold water into the ear to cause vertigo. The hot water tricks the nerves in the canals, and the eyes start drifting away from the hot water. You then realize your eyes are drifting, and you snap them back to where they were before (towards the hot water), a very quick movement called "nystagmus". Cold water does the same thing in the opposite direction. This will make everybody have vertigo. So why would a hot shower give you vertigo? The hot water may simply be activating your nerve in this way. Your "cold" shower may actually just be room temperature and not actually be as cold as necessary to do the same thing in reverse (icy cold). But i don't recommend taking an icy shower just to test it out. The first thing i recommend is to avoid putting your ear in the water, or wear earplugs. This might fix the problem right away. If it still happens when your middle ear stays dry, then you are probably extra sensitive. I would experiment with different ways to avoid getting hot water near your ear. Of course another possibility is that it has nothing to do with the water, but i would expect you to get vertigo at other times, too if that were the case, like picking things up off the floor. So this really sounds like the temperature is mimicking this neurology test, and you just need to stop putting your ear through the experiment every time you take a shower.