When it comes to the self the concept of brute force doesn't really exist. You can't tie yourself down or force yourself to do a good deed. You have to just do it. I think inner-strength is about self-enforcement. Inner-strength is placing your beliefs in practice and ensuring enforcement of same.
Right up front you have to know what your beliefs are. Without that you can't stand for anything. And you especially can't hold others to a standard. But also I think you also have to have boundaries. You have to draw the line where in a world full of people each individual will have their own set of responsibilities and often roles are defined in a relationship so responsibilities too are shared.
Step 1 in developing inner-strength is knowing your beliefs.
Step 2 is articulating the set of responsibilities that stem from your beliefs.
Step 3 is knowing the parties involved in a situation and clearly articulating who owns what responsibilities.
Inner strength is about maintaining integrity of the self. Knowing what and who you are so clearly and with such conviction that nothing can disturb that. And that is especially important when dealing with people. Knowing that line between yourself and others is crucial. I think there are several kinds of strength and can come from different places. It could be maintaining your physical integrity or keeping yourself focused and free from distraction. It could mean standing by your beliefs and not allowing others to push you around or choose your beliefs for you. It could also mean keeping your cool, staying centered, relaxed and focused. Not allowing others to effect your emotional center.
It's all about staying on target, keeping focus, not losing sight of what you belief and want to accomplish. It is resilience. Grit.
But thinking doesn't get anything done so it is time for action. Like responsibility action is split between the self and everyone else. To be strong you must live by your beliefs. Do not be a hypocrite, do not be a push over. And it swings both ways: do not take on more than you are responsible for. Own what is yours, fully and completely, but no more.
Step 4: Do your part and make it happen.
Step 5: Hold others to their part.
If you can manage all of that the tricky bit is how to manage other folks. If you want respect or to be perceived as strong you definitely have to hold folks to their responsibilities if they are unwilling to do it themselves. A lot of people avoid responsibility (iow being a good citizen) and many others will take advantage of as many folks as they can. Don't let these people burden you.
For most of my life I've followed the creed of let people be. Unless people's behavior was egregious it has been trivial for me to ignore people's childish behavior. I can let things roll quite easily because those things don't stand in the way of what I'm getting done. And I also like to keep my cool. It seems more platonic and high ground to lead folks to water rather than dunk their head in it. But I don't believe society has a lot of respect for that. I think it is perceived as weak. You also can't club people over the head. So this calls for soft power and strong wills. I'd like to find a good middle ground. But for now it is sufficient enough to say that other people need to be held responsible for their deeds and responsibilities.