As you are only connected to a central pivot point (the nema 23 motor), I don't think you will be able to dampen the lateral vibrations at that length and weight. At a minimum you need a second point (such as affixing your pivot point to a stand and having the motor positioned further down the arm running around a large gear).

 

What are you running for firmware? Besides changing the printer mechanically, you could just try reducing the jerk. It might also help to reduce the current of your stepper. Another option would be to change the size of your extrusion. The picture doesn't depict much and it is hard to see how hard it would be to implement, but if from the perspective in the picture, if the extrusion was made wider, the maximum strain would be reduced because material on the sides would increase. Similar to the concept of I-beams.