Calculate Input Force Based on Current Velocity and Max Movement Velocity Whilst Maintaining Momentum

Okay, So I am struggling to wrap my head around the maths involved. Basically what I want to ac$$anonymous$$eve is when a character is in the air, the character can manoeuvre about using input up to a specific velocity in all directions.

For example, Imagine the character is launched forward at 100 m/s and is facing in the direction of travel. Assume standard WASD controls. The character can move a maximum of 5 m/s under its own power:

• I push the W Key. The character will continue moving forward at 100m/s and continually decay based on drag and will not increase momentum until the velocity in t$$anonymous$$s axis is below 5m/s

  I push the S Key, The character will slow down at a rate of 5m/s/s in conjunction with the drag coefficient.

  I push the A or D Key and the character will move left or right horizontally until the character reaches 5m/s in either direction and will not go faster than 5m/s w$$anonymous$$le simultaneously maintaining momentum in the forward axis.

I would like t$$anonymous$$s principle to work for all direction permutations and I'm not quite sure what functions I would need to use. I do believe it is going to be a case of taking the current velocity and crossing it somehow with the player's input movement vector * max speed with the current velocity.

Here is what I have as current:

 // We are in the air
 if (_moveInputVector.sqrMagnitude > 0f)
 {
                     
     targetMovementVelocity = _moveInputVector * MaxStableMoveSpeed;
 
     // Prevent climbing on un-stable slopes with air movement
     if (Motor.GroundingStatus.FoundAnyGround)
     {
         Vector3 perpenticularObstructionNormal = Vector3.Cross(Vector3.Cross(Motor.CharacterUp, Motor.GroundingStatus.GroundNormal), Motor.CharacterUp).normalized;
         targetMovementVelocity = Vector3.ProjectOnPlane(targetMovementVelocity, perpenticularObstructionNormal);
     }
 
     Vector3 velocityDiff = Vector3.ProjectOnPlane(targetMovementVelocity - currentVelocity, Physics.gravity);
     currentVelocity += velocityDiff * AirAccelerationSpeed * deltaTime;
 }
 
 // Gravity
 currentVelocity += Physics.gravity * deltaTime;
 
 // Drag
 currentVelocity *= (1f / (1f + (Drag * deltaTime)));

Can someone help me out here and suggest what I need to do in order to conserve the momentum of the rigid body w$$anonymous$$lst honouring the character input up to its max speed in any direction?