Stability is endurance rather than force, and its significator is Saturn, the planet of limit, duration and what holds.
Saturn is cold and dry, the greater malefic, the slowest of the seven and the outermost of the visible planets, and in the classical cosmos it marks the boundary of the planetary spheres. Everything about its signification follows from slowness and edge: time, age, structure, walls, bones, patience, poverty, and the capacity of a thing to last. The tradition calls it a malefic because what limits also deprives, but it does not treat those two significations as separable, and a chart that wants durability has to accept Saturn to get it.
The four fixed signs, Taurus, Leo, Scorpio and Aquarius, carry the same quality by sign. They stand at the heart of each season, when the weather of that season is settled and fully itself, and the tradition reads them for holding a position, resisting change and seeing a thing through. Saturn rules Capricorn and Aquarius, and Aquarius is one of the fixed four. Saturn is exalted in Libra, which is not.
A reading distinguishes stability that is chosen from stability that is imposed. The same fixity that lets a person hold a course is what keeps them in a position they should have left, and traditional practice takes the difference from the condition of the significator rather than from the placement alone.