The fear of losing money will cause regret when you win a bet. In my opinion, it is just an illusion that crosses someone's mind because they are unable to bear the loss when they lose. In fact, if he was able to think objectively, winning and losing are part of gambling, maybe this small win would not have happened.
Obviously, if a fearful gambler that is very scared of losing money win a bet he will not be contented with the value of the bet he won because the amount might be decent to him or kind of too small for him, and he's the reason why he won that decent or small amount of money. However, he's not afraid of winning but afraid of losing and it's not supposed to be that way. If you have the mindset of winning you should also have the mindset of losing. That's why a gambler should risk what he can afford to lose.
If he's scared of losing money, it means he can not afford to lose them which is why he stake low amount and IMO he shouldn't feel regret even if he wins a small bet (money) because he couldn't take the risks of staking high cause he can not afford it.