Run back to the scene of your demise to reclaim these lost souls. Die, and you drop all your unspent souls on the spot. Use those souls to level up your character, your gear, etc. There is one currency in Dark Souls, souls. It presents you with a brutally honest fighting mechanic, lets you take as many swings as you want, but tries to take everything you've earned when you falter and die. The hallmark of the Dark Souls series is its difficulty. I entered the world of Lothric eager to tackle the challenges ahead, excited about the journey before me, and undaunted by haunting tales of an unforgiving gaming experience. Let's face it, everyone's journey through death and dying is a personal one, and just because I'm not the first, doesn't make my experience any less authentic. But the longer I spend with Dark Souls III, the more I am convinced that there really is no other way to frame the discussion for a first time player to the series, except through these stages. Initially I walked away from writing this review from that perspective, it's been done before.
A quick google search will yield that I'm not the only reviewer to feel this way. Even in my earliest playing hours with FromSoftware's Dark Souls III, I couldn't escape the feeling that the experience of playing this game is like moving through Kübler-Ross' 5 stages of grief.