I have Astral Heron on the battlefield. My opponent pushes my battlefield and I react, playing Retreat on my Heron. Does my next card gain the discount?

No. Heron's "if I'm at a battlefield" is part of its trigger condition, and that condition is checked after your first card of the turn fully resolves — by which point Retreat has already recalled Heron to hand, so the ability never triggers.

The trigger condition is evaluated only after the inciting event is fully processed (Rule 383.2.c). Since Retreat resolves and returns Heron to your hand before the condition check happens, Heron is no longer at a battlefield and the conditional fails. The discount only applies if Heron is still at a battlefield when the first-card trigger is evaluated (FAQ #11610). You'll need to get Heron back to a battlefield and play a first card that doesn't remove it to get the discount.