I don't think it's so much him regretting the one night stand. It's more him regretting not chasing after the girl when he had a chance. He didn't want to just have a one night stand, but he didn't fully understand his feelings and what he should do with them (being a full-time man). So instead of doing what he felt, he ran because he had no idea how to handle these feelings. Now he regrets running and knows that he has wanted to be with this girl all along, but it's too late and now he is the fool.
I don't think it's so much him regretting the one night stand. It's more him regretting not chasing after the girl when he had a chance. He didn't want to just have a one night stand, but he didn't fully understand his feelings and what he should do with them (being a full-time man). So instead of doing what he felt, he ran because he had no idea how to handle these feelings. Now he regrets running and knows that he has wanted to be with this girl all along, but it's too late and now he is the fool.