As a trader, what's something you wish you knew when you took your first trade?

The basic knowledge about stock market. How market works. There is real truth about swing trading.
As a trader you have to put strict stoploss. Stoploss heating again and again mean you have to study more and more and find out where you are comfortable to take trade.
mean find out the price at which you can initiate the trade. There number of ways to take trade. Some choose price action, some choose indicator based, some swing based. Some wave based. Find yours.
Must knew position sizing so that betting goes in structured way. Else the concept of money management will suck.
Knew to control emotion like fear, greed, etc. If not read/ learn if possible how to control emotion.
Intraday is all about timing. So if you are thinking of intraday then look yourself to time the market.
Moreover fundamental analysis is also important if you don't know technical. So combination of both makes superior out of all.
Most important, don't look for overnight success. I will make so and so and so percentage. That doesn't work of Excelsheet extension. It take years to build career into the stock market. So give sufficient amount of time and learn and then thinking of earning.
Happy trading!!!!