What if trust is learned as a whole, when young? Your parents are a force of nature. [...] If they're capricious, unpredictable, and worse, malevolent, then that's your emotional estimate of the universe. It's not that you don't have self discipline, it's that you live in a malevolent, unpredictable universe that you rightly don't trust. Or so it seems to you.

Yes. People bring many aliefs from their childhood; predictability of the universe is probably one of them.

If your model says that one marshmallow is sure, but two marshmallows have probability smaller than 50%, then choosing one is better. If your model says that you cannot trust anything, including yourself, then following short-term pleasures is better than following long-term goals.

How can this model be fixed? It would probably require a long-term exposure to some undeniable regularity. Either living in a strict environment (school? prison?) or maintaining long-term records about something important.