Considering risk and regret, which do you think will be the most painful for a gambler?
The risks and regrets are subjective, so it depends on the gambler and the context in which they are used, and they could be both painful with almost the same magnitude.
Risk: Wrong risk will definitely be painful especially if the gambler loses and could be aggravated if a huge amount void of proper management is involved. While the right risk is less painful because it can't be associated with a big risk and the gambler must have understood the risk he is taking.
Regret: It may also come in several ways and magnitude. You might regret if you take a risk when the outcome is negative and you might regret when you refuse to take a risk when your intended option and outcome come as you wanted.
Above all, risk and regret can be used interchangeably in gambling, notwithstanding, it's better you missed the opportunity rather than regretting later out of mistakes and senseless risk-taking.
You are correct that risk and regret in betting are highly subjective and can be equally painful depending on the situation. However, one must keep in mind that both can be steered clear of, or at least minimized, with a more measured and thoughtful approach.
Risk-taking is part of gambling, but it can be curbed by being well informed about your limits and proper management. Regret typically results from imprudent actions; most of the time, regret stems from acting without thought. Hence, it is wiser to be prudent in decision-making than to have avoidable regrets come later on a platter.
Ultimately gambling should be, done with a full awareness of the downside and not just chasing opportunities uncalculated risks.