You most likely bought the wrong tool for this particular job — and it's a very common mix-up. Pink-to-red flat marks after acne are post-inflammatory erythema (PIE): dilated or damaged tiny blood vessels left behind by the inflammation. Brown marks (PIH) are excess melanin. Dermatologists coined the PIE term precisely because the two get lumped together and treated the same when they shouldn't be (https://pubmed.ncbi.nlm.nih.gov/24062874/).
Vitamin C, arbutin, and most "brightening" actives work on the melanin pathway — they inhibit pigment production. PIE has no excess pigment to inhibit, which is why two diligent months showed you nothing. It's not your technique or the serum's quality.
What actually helps PIE: time (vessels remodel on their own — most marks fade over weeks to months), not picking or squeezing (each round of trauma resets the clock), and daily sunscreen, since UV slows the skin's remodeling and can deepen the contrast. The treatment with the best track record for stubborn PIE is vascular laser (like pulsed dye laser) in a dermatologist's office — that's what the studies behind the term actually tested.
So: keep the sunscreen, keep hands off, and let the vitamin C earn its place on other merits (tone, antioxidant support) rather than this one. If specific marks are still bothering you in six months, a laser consult is the evidence-backed next step — one quick derm visit beats another year of serums aimed at the wrong target.