Try chewing natural foods in that area to create force links: ANgle a ROUND raw carrot across the higher cusps and practice rotating and angling the carrot to encourage the cusps to align better with upper and lower teeth. CHew on the premolars to create compression which will create tension in an anterior-posterior direction along the mandible, which will "stretch" it forwards, making more room. Do not reinforce the problem with a retainer. Bite into foods with canines and pull, tearing the food, and helping to create tension on the mandible. Your ortho is an idiot for not showing you how to chew and for not observing you chew.
Thanks. I am going to chew the way you said. I tend to chew without any sideways rotation just going up and down in the same spot. Do you think the fixed retainers are a problem?