Being disciplined is something that is hard for everyone but most times some people mistake being disciplined with being principled.
While growing up, we all have the way we were thought how to do things and with time, we create some principles for ourselves that might either be good or bad. And with time as we get wiser and learn more, we find out that some of our principles are unhealthy for us and even others but we find it hard to let go and that's because we are not disciplined.
The best way to stay disciplined is to put yourself in-check. What I mean is, tell someone you want to forgo an unhealthy habit and ask them to help you. This is only hard for people who have pride and those who are not willing to change.