I too have essential thrombocythemia. I am 43 and my platelets are between 577 and 730. I too had a bone marrow test which confirmed that I had ET.
Why are you taking medication? Did you have a thrombotic event, ie, a heart attack or stroke? You should read up about ET as thinking now is that if you have a platelet count of less than 1500 and are low risk (ie, young and have not had a thrombotic event) then you should be treated with low does aspirin only. I can't take aspirin so I take Plavix instead. My platelets were picked up on a routine blood test 18 months ago and I don't have any symptoms. How high are your platelets?
I have been through all sorts of hell worrying about platelets, but feel a bit more relaxed at the moment. I realise though that if my platelets started to climb, I would be totally freaked out again. It is stressful to go to the haematologist and I worry the night before. They don't know why people have ET and of all the MPDs it is the best one to have - although I would rather not have any and not know anything about it.
I would be interested in knowing how things are - if your platelets are going up, maybe it is time to have another BMB - but hey - I am just a patient, so be guided by your haem.