Most people tend to look at the Mind as the great Problem Solver, and hence all the discussions about Man's abilities and strengths are predicated on the Mind in such a role. However, from my own Life experiences, I found that the Mind is the Problem Creator, and if one learns to 'quiet' it, You have that Bedrock you are looking for... within yourself. Just saying.

A fun conundrum, and one I ponder from time to time, but I've decided that as wonderful as that inner locus of control is, you pretty much have to acknowledge that *at scale* you don't make things happen. Natural disasters, market forces, a relationship dying--- you've got no control over that. So when you're faced with the abyss you pretty much have to just size it up, and size up the forces you can rally against it, and decide then and there whether "losing it all" is going to make you act in a way you're not proud of. 

Failure can only be total and absolute if you allow it to be. If your self of self relies on you being the absolute master of the universe, then you my friend are fucked. You can only do what you can do and make sure that you're proud of the effort, that you gave it your best. 

When overwhelmed, I tend to think about how many people are suffering from such easy-to-solve problems right this moment.

If my current battle is too hard for me, I say to myself, you could always sell all your belongings go help these people more directly, working in a soup kitchen or moving to a third world country.

That inevitably brings me back to, "but the best way to help them is to change the world at a more fundamental level." It doesn't negate defeat / weakness, but it does help me not lose my perspective.

How about actually trying this test against successful people?  Ben Franklin, Bill Gates, Elon Musk--what are their greater causes?  I'm skeptical that this is as true as you say.