Read it slowly and the difficult word is not rest. It is alone.
We are fine with God as one source of steadiness among several. Add Him to the savings, the relationship, the plan that is mostly working, and the general sense that things will probably hold.
The psalm removes the others. Not because they are evil, but because they cannot bear the weight. A rock does not shift when the weather does. Almost nothing else you are leaning on qualifies.
You usually discover this by accident. Something you counted on gives way, and in the space that opens you find out what was actually holding you up. That discovery is painful and it is not a punishment.
He alone is my rock and my salvation, my fortress. Three images, all of them immovable, all of them saying the same thing. Rest is available, but only where the ground does not move.
