Most of us have heard stories of how serving in the army makes for an amazing discipline in a person. It seems to be true for most of the world's armies, even the ones that reportedly don't do much active training: a rebel goes in - a confident and purposeful young person goes out.
It's easy to understand the direct reason: the army pushes a recruit through a series of very hard challenges, which apparently break the recruit's spirit, breaking them off of their old mindless habits and allowing spirit to reform under a new paradigm, which includes, among others and in layman's terms, a concept of "whole-assing the thing you deem worthy doing". This is achieved through severe pressure from officers, who take it as their job to punish the soldiers enough to allow for paradigm shift.
Is it possible to achieve the same level of discipline alone, without such a hard pressure from outside? If so, how does one achieve that? Good discipline is without a doubt a desirable trait to achieve.
As someone who has been through basic training, I don't think of it as something that is done to someone. The recruit is still an active participant in it. I've seen many people wash out because they didn't realize that the military doesn't create that "whole-assing" spirit in you, but gives you the environment to create it in yourself. This bodes well for the idea that one can "do it alone".
However, I don't think you actually can. It's natural to reach periods where one can't find the motivation to go on and the temptation will be to give up. Unless you already have the dedication you're trying to create, it's very likely that you won't make it through those periods. This is where I don't think anyone can do this alone. People need someone to drag them through these periods. That's the function that basic training serves: it makes it very hard to give up.
In the end, I think it is possible to create that drive without joining the military. I don't think it's possible to do it alone. Hopefully, you can find someone to help, someone who will help push you through the low motivation periods and keep you going.