Thanks for playing!
Fuel regenerates pretty quickly, but it only regenerates after not being used for a couple of seconds which isn't really explained in-game. Hoping to address this soon as well as add a visual indicator of when the regen is kicking in.
Artifacts (equipped at the reliquary) can also help address that as well as fuel pickups (the lightning bolt things) in the map. I'm seeing that these mechanics really need to be explained clearer in the game somewhere