I have been self-medicating for a medical condition for some time now (easy to do without prescriptions in most Thai pharmacies; all I have as proof is a single script I got from a doc in my home country before moving here a couple of years ago). I never thought I would need to have a prescription here. But now I am facing a situation where it will help me enormously if I can get a doctor's statement to the effect that I have indeed been suffering from and treated for the condition over the last 2 years. If anyone knows a doctor who will have understanding for my situation and won't break the bank, please msg me. I very much appreciate, as I am desperate for help. Thank you (Bangkok area)

Nobody can tell you in advance what a particular doctor will or will not feel comfortable certifying to. You'll have to see a doctor, provide a full medical history, undergo a physical exam and then it will depend on whether the physician feels reasonably confident about the diagnosis and its duration (based on history and exam findings).

Since you had a prescription from a doctor at home 2 years back, is it possible to get those medical records or a brief note from that doctor? As that would help establish timeline.