Gambling, sports, or slots has its downside. Like what Op just felt in sports can occur in slot games as well. But in a different context. Slot game happens in a way the gambler will argue with his instinct on whether to hit the next button or stop. He'd imagine he could win on the next one, then try again, if he wins, he'd want to try again, believing that his idea worked. Although sports gambling allows the gambler time to watch the game without doing any other thing, other than thinking, slots put the gambler on steady thinking and gambling. He has no time to rest, he's free once the session is over. Sports betting and slots seem almost similar, and none is easier than the other.
I understand that knowledge and analytical strength are a plus for sports gamblers, but as you said, it doesn't guarantee a win. Like slots, being a professional doesn't mean you'd win. Gambling in any form can't be understood completely. What led Op to withdraw before the final whistle was blown was self-doubt. He didn't want to lose out completely. He wasn't wrong, he got what he wanted. Additionally, he got greedy when he realized that he could have won more if he left the funds untouched. It's a "had I known, I knew" situation, and the player can think about this for a long period.