Just downloaded the most recent free build of the game and I can't seem to heal at all unless I save the game.

The game isn't super clear about it and I only figured it out myself through experimentation, but when you defeat enemies and collect the orb things, a green bar below the blue orbs under your health bar will fill up. When that bar fills all the way up, a green orb will be filled next to it. 1 orb heals 1 health bar tick, and you can have up to 2 orbs on hand.