Quick mechanics question

A mass on a spring : like this, M the weight of the mass, k the constant of the spring, d the compression distance. And I just noticed that I didn't write that E1 = E2, E2 being the total mechanical energy when the spring is compressed.

I'm solving for the compression of the spring d, first with the mechanical energy of the system, then with just the forces, and I'm missing a factor of 2 when solving with the forces.

What am I missing ? Thanks

Edit : When solving with the potential energies, I get the max compression, not when the mass stops moving. When solving with the forces, I get the compression when the mass is stable, not moving.