Every time I reflect on myself, the urge to practise polygyny disappears. What does that mean?

When a wish fades as soon as it is examined calmly, that often means it was driven by a passing feeling, not by real need or readiness. An urge that only lives when emotions run high and dies when you think clearly deserves attention, not dismissal. Keep reflecting: write down the real reasons for the wish and what you hope it would solve. Often it points to something that can be improved in yourself or your existing marriage. Invest your energy there. If, in calm moments and after istikharah, you are still sure you are able and your intention is right, only then consider it seriously. A clear mind is a better adviser than a turbulent desire. See polygyny readiness tool.