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.
I think you wrote something a bit wrong above my friend, because the fear is clearly caused by the worry of losing while above you wrote that a gambler feels afraid when they win, that's a bit hard to understand and I'm sure you wrote it wrong above but it doesn't matter my friend. Another thing you are right about your opinion in the last part that losing is part of the game where the results can never be separated at any time, so gamblers really don't need to be afraid of losing. Usually I call gamblers who are afraid of losing are losers, the point is if for example you are afraid of losing then in my opinion there is no other way than not gambling at all.