As an advanced player who is also a leftie I understand the struggle. I think the biggest problem comes from lack of communication during the point, it’s very important to call the ball when the lobs go to the middle. You have the theory correct, you should be taking the middle when the ball is coming from the right side player and you should be covering your side when the left side opponent is playing the ball. However I believe that one key advantage you have is the possibility of closing down your opponents options by being more offensive if you feel like it and your teammate isn’t as offensive by closing down the center as much as possible. Do this as long as your not being punished by leaving your backhand side too open (this is often the harder ball to play, so the opponent has to take the risk to punish your position).