bitcoin slowly went down this year, from almost 20k USD last December and went down at a significant price
what might be the reason why bitcoin's price is going down?
what are the major things that can greatly influence bitcoin?
answers would be fully appreciated

Usually it is the FUD that is affecting the price of bitcoin. Usually when there are negative news around related to crypto the market reacts negatively and there are also speculation that maybe whales are manipulating the price of bitcoin because they have the capital to do so.