How can you be self disciplined? I don't think there is a how....you just do it. Does this sound unhelpful? Let me explain.
When people say they struggle with self control they often think that there must be something special that they need to do and that if they knew the secret then self control would feel easy. If you focus on the how and the why of your situation that won't help you DO what needs to be done. There is an analogy attributed to Buddha where he says it's as though you've been shot with an arrow and instead of pulling it out you want to know who fired it, what direction it came from and what's it made of.
So if you find it hard maintaining a healthy diet or keeping an exercise regime it doesn't matter...do it anyway!
The thing is that once you ingrain a new habit it becomes easy. Take up running say and after a couple of weeks, it's no longer a struggle to do it. People are often impressed that I practice Tai Chi twice a day, every day but really once the habit's been formed it's just what you do....unless you don't. All of us will have times when we slack off. We then start to form a habit of not doing the thing and it takes an effort to start again.
You need an attitude of going easy on yourself in spirit but hard on yourself in action. What I mean by that is, don't beat yourself up because you've eaten a whole packet of biscuits just get back on the path of not doing so.
When I trained as a hypnotherapist I was taught that control is a constant, you're either in control of everything or you're not. This means that if you take control over one thing you take control over everything.
As always, I'm going to steer the subject towards Tai Chi! Most people find establishing a regular Tai Chi practice difficult. This is normal. It's difficult but do it anyway. Begin by practicing a very small amount regularly, maybe 5 minutes every day. The important thing is that you're creating a new habit. Everyone has 5 minutes to spare! Keep it up and notice how maintaining the practice gives you self discipline in other areas of your life.