Self-discipline is about inhibiting impulses in the moment. Such as making yourself do something you don't want to do at the moment, but you know it will be better for you long term.
Self-regulation is about reducing the frequency and the intensity of the impulses by identifying what's happening.
So you can say that self-discipline is about consequences of the action caused by these impulses, self-regulation is about not having to deal (so much) with impulses that won't help you.

With self-regulation there will be less need for self-discipline. Self-regulation is often better than self-discipline, because with self-discipline, it might not actually reduce the impulses, so you will have to keep inhibiting the impulses and they may even become worse.
Because as long as the brain thinks it's needed for the health and survival of the individual, it likely won't stop giving these impulses.

Which comes to another bad thing about self-discipline, that your brain can actually be right (in some way), such as in a situation were a person is being treated poorly, then to just inhibit impulses of the brain won't help the individual and will only make the situation worse.