Partial answer
You are:
A cup of coffee, caffeine
You are the hero of my every day
scaring the black demons away.
Having a cup of coffee in the morning wakes you up, chases away the sleepy feeling
In tiny circles, round you march,
Tiny circles, coffee cups or espresso cups. Coffee “marches” to fulfill their duty in tiny circular vessels.
over rows of weapons, strung in an arc.
Not sure on this one yet
Your own weapon is gentle, does not destroy,
but fills me up with frothy joy.
Caffeine is gentle compared to other drugs, and does not destroy your life when addicted to it. Frothy joy might refer to a latte and the mood altering one experiences when caffeine is being processed by the body.