Forgive me, b/c I haven't read up, but you keep your C in the fridge, right? Also acids work better/eat your face less if you gradually lower your skin PH via cleanser/toner, ideally to at/around 5.5. My favorite toner for this is the Mizon AHA/BHA, but it was out of manufacturing for a while.I'm thinking of trying Timeless 20% Vitamin C + E Ferulic Acid Serum. The E and Ferulic acid make it much more stable so it doesn't oxidize as quickly. It's a dupe for a Skincueticals product that costs $160 fucking dollars. I'm worried though because the ph is low so it might eat my face.
https://www.timelessha.com/products/20-vitamin-c-e-ferulic-acid-serum-1-oz