You’re playing the Sorcerer, so not your call. The DM decided, quite reasonably, that a warhorse dashing blindly in the dark would likely injure themselves. The DM probably also should have called for a couple of Animal Handling rolls (which I’m also guessing your character would have trouble with).It was never broken leg, because you control the warhorse.
The Steed doesn't need darkvision, because you control it and you can see on Darkness.
You are now restrained below the horse. With an 18 Dex, 16 Con and 16 Cha, I’m going to guess your Str is 8 and you aren’t trained in Ath. At a -1, you aren’t getting out from under that horse anytime soon (DC 13). Even if you hadn’t already blown 5 of your Sorcery points, Magical Guidance is unlikely to be much help.
Your continued existence depends solely on whether you have enough Shield spells to survive until your party successfully defeats the encounter.