There are many specific things that traders worry about, but some of the most common include:

Losing money. This is probably the biggest fear of all traders, and for good reason. Trading can be a risky activity, and there is always the potential to lose money. Traders worry about losing money on individual trades, as well as losing money over time.
Making the wrong decision. Trading is a game of probabilities, and there is no such thing as a sure thing. Traders worry about making the wrong decision and losing money as a result.
Missing out on opportunities. Traders worry about missing out on profitable trades, especially if they see other traders making money. This can lead to overtrading and making impulsive decisions.
Not being good enough. Traders worry that they are not smart enough, experienced enough, or disciplined enough to be successful traders.
Being judged by others. Traders may worry about what other people think of them, especially if they are new to trading or have a losing streak. This can lead to anxiety and stress.