I lift. I eat right and I take cold showers. Those aren't what makes me disciplined, those are consequences.
If what you get from reading Meditations is that he sleeps on the ground, then you have already lost. Read it once more but look to the message of providence. I know it's a Hitler word, but remove it from the one man and see what providence is. There is a good in all men.
Meditations is not about the 'punishments' he puts himself through. It is about the mindframe he has. Read Epictetus, Discourses. It is not the man, the soul, or the status. It is his actions. A man worries only about what is his control. Recognize when you're feeling any emotion and say "do I control what causes my problems". If yes, then change it. If no, then stop worrying.
To develop this habit you must make an effort to recognize emotions first. They govern us at almost all times, so you should nearly always have something to observe. When you say that you aren't feeling anything right now, then you know you're wrong and this is something to overcome.
After you recognize them, analyze them. Do not judge your emotions. They aren't yet in your control, so you do not worry about them. You try to change it to see how much control you have. From there, you can develop discipline.
I will reiterate. Stop at least once a day and say "what am I feeling". After a week say "why do I feel it". Then ask "do I control the cause".
A good way to start is to have a habit that you do every day. You sit down and read a chapter. It's short and provocative. You then think, how do I feel now. Grow that thought to govern you throughout your day.
You will likely never gain full control. We are humans. That isn't in your control. Do not worry about it.
The cold shower is simply so you can say "why am I adverse to this shower" "how does it make me feel" "do I control the feelings that lead me to not want to take it". Then you get in, shower, and learn you do control it.