I have et for 8 years. since i was 22. my plt count was over 1m most of the time. in the last two months my plt count is 500k how can it be?

By ET, I presume: you mean essential thrombocythemia, an uncommon disorder in which your body produces too many platelets. It likely results from a gene mutation. If your platelet count is usually around 1M, I would first suspect a lab error. Repeat it now. Discuss with your hematologist. ET would not be expected to spontaneously improve; some complicating or interfering condition might be occurring. Good wishes:)