I want to know what you think, what affects the bitcoin price, so we can find out when to sell and buy bitcoin, because now the bitcoin price drops again 
We can not just mention one factors that caused the unstable price of bitcoin, because in reality, the price of bitcoin is involving many factors inside it. But, i think, the most things that caused the unstability of bitcoin's price is the amount of supply and demand. There are several causes which effect bitcoins. Any false news or negative impact can take it down,
Where good news like more investors, increasing the demand can take bitcoins up.
But we should not disappoint with it actually when the price drop it become perfect time to buy more because when the price decrease it increase double in future.