Played 6 hours so far. I haven't banked any souls, since I keep dying.
I'm at the first boss (Last Giant), and I keep fucking dying.
Have I broken the game? Like, did I totally fuck up by losing those souls? All the enemies have disappeared.
No. You'll be fine just spending the souls you get from eventually beating bosses. Try not to lose those. You can always do coop to farm for more souls too.
In previous games you could just grind normal enemies forever, but there's limited enemy spawns now. But still, losing your souls over and over is a pretty core component of how the game is supposed to play out. You'll be fine!