Peace of mind is very important and can help us become a better trader if we don't take money from people to trade in the market. I have seen friends entering into debts because they are too confident about the market and their strategies, going far as getting money from families and clients, trading for them why getting their cuts. We ought to learn the habit of gambling with our own funds so if anything happens along the way, we can bear the loses without having to be panicking about paying clients money. Experienced traders are attest to this which has ruined the lives of many traders because they believe they can trade with confidence making profits from the market.
One of the reasons why peace of mind is very important for a trader to have is because trading is a decision-making activity where a calm mind can prevent you from feeling doubtful when you want to open a trade, on the other hand, I think feeling confident is a good ability that a trader must have, but that's also if for example you trade using a strategy or skill and not acting carelessly, the benefit is so that we don't miss the right moment when we should have opened a trading position.
Trade with the amount of money we can afford to lose? of course, because basically trading is still a risky activity where when you can't accept losses, there is a possibility for you to lose more money due to chasing previous losses, and actually trading with an amount that is ready to be lost is also one of the best plans that will help us to be calmer when trading.
The point is, if for example you do trade by relying on a strategy that you think is good, then there should be no doubt for you to open a trade, especially when you find the right moment, and if for example in the end you lose, then that's a natural thing.