Applying a counter force to cancel out velocity?

I am pretty certain my math here is correct, but I have some lack of understanding of how the unity physics engine works internally. Essentially I want to cap the velocity’s magnitude at a certain value by applying a counter force in the opposite direction with the magnitude of the excess force. I understand I could use a clamp, but I don’t want to for the sake of an accurate simulation. My code:

private void AddClampedForce(Vector3 force) {

    float sqrMaxSpeed = maxSpeed * maxSpeed;
    if(velocity.sqrMagnitude > sqrMaxSpeed ) {
        float forceDifference = Mathf.Sqrt(velocity.sqrMagnitude - sqrMaxSpeed);
        Vector3 counterForce = -velocity.normalized * forceDifference;
        rigidbody.AddForce(counterForce, ForceMode.VelocityChange);

        // Usually Gives 14 or so, ocassionally higher if I hold two buttons down and move diagonal.

I am pretty sure that your math is wrong

Mathf.Sqrt(velocity.sqrMagnitude - sqrMaxSpeed);

Kudos for comparing squared magnitudes and saving one or two sqrts, that’s what I always recommend. BUT the line above does NOT equal (velocity - maxSpeed)
Start there, see if the results improve