LSD (Lysergic acid diethylamide)
Physically I’m burning, but not on fire,
LSD is AKA acid, which burns
Habitually referenced by the two double five,
It may be an effect of blurred vision where the user sees doubles. Or the drug causes the user to collapse on all four limbs, which is two sets of hands/feet each with five digits
Coming from the sky, ground, and water in time,
Made by reacting the colorless Diethylamine liquid (like water) with Lysergic acid (made from natural components)
How about my opposite, he’s simple as that,
A non-LSD user probably leads a simpler life
Erasing - the walls, Displacing - the falls,
LSD can erase memories or cause the user to trip and fall.
Most often, you’ll find me on the streets, on a score.
You can get LSD on the streets, only if you are successful (a score)
Lastly, from the title
Capital letters spell LSD