Your menu is simply taking on styles from the higher screen setting. Also, some of your elements within the Nav Container don’t have styles applied to them at all. This is actually one of the challenges in site building that troubles many. When the content increases there needs to be a guide or piece of paper with naming conventions and sizes written down. Try to make sure you have everything labeled then you can stay on top of what you did on desktop, so you can know why mobile is effected.
You’re pretty far along, so I wouldn’t worry too much about it now. Just go to mobile view and take out, move or resize what you don’t want. The desktop and tablet view will stay intact. Styles move down not back up. Just open the menu in mobile and click on each link section to add padding or margin. Otherwise, you will be in for a long haul to get everything just right I wouldn’t go back to restructure if I were you. Take care.
Also, think about it like this… No real customer is going to - Open your site on a PC, then take the browser and minimize it… then resize to the size of tablet and read the site! Who opens a site on a tablet and then picks up their cellphone and then put the tablet down? Not likely… your fine I like the site.