Firstly, we're sorry to hear that you're experiencing such long term negative effects - and unfortunately far from the first we've heard. HPPD is not well understood (especially when start of symptoms is when the brain is still developing as a young adult); consequently, the clinical treatment options are scarce and inadequately studied.

To start on simply keeping it from getting worse: you're definitely on the right track with ceasing use of hallucinogens. Anyone with lasting detrimental side effects from taking psychedelics should probably not take those psychedelics in the future. There is some anecdata indicating the other stimulants, such as common ones like caffeine and nicotine, may also exacerbate symptoms.

There are a few case studies out there in the clinical research indicating off label treatment using Lamotrigine has been successful. It might be worth seeking out a psychiatrist who has worked with HPPD specifically in the past to tailor a treatment protocol for you. Treatment of any co-occuring anxiety or other mental health issues has also been indicated to reduce severity of symptoms.