HT1222 The music in my iTunes library run together and play at the same time.

Question

The music in my iTunes library run together andplay at the same time.

Answer 1

The "bar" at the bottom does not say anything about all of your songs being on the iPod.  It just shows you how much of its storage capacity is being used, based on what is on the iPod.
If you see "On This iPod," that means the iTunes sidebar (along left side of window) is not visible.  This is easier to describe with the sidebar visible, so from the iTunes menu bar, under View, select Show Sidebar. 
Select the iPod in the sidebar, under DEVICES.  There is a bar of buttons near the top, starting with Summary.  Click on Music next to Summary.  This is the iPod's Music screen, where you tell iTunes how to sync songs to the iPod.
Check the box for Sync Music.  You can choose to sync Entire music library, if (?) your iTunes music library fits on the iPod.  With this setting, your entire iTunes music library syncs to the iPod, and new songs get synced automatically.  A 7th gen iPod nano is 16GB, so your iTunes music library would have to be less than 16GB (not counting non-music content on the iPod).  Otherwise, you can choose to sync Selected playlists, artists, albums, and genres.  Then, select (checkmark) the playlists, artists, albums, and/or genres you want on the iPod from the lists below.  If you want newly added songs to sync to the iPod, they must be part of your selection.  For example, you can add them to a playlist that you have selected for syncing to the iPod.
When you click Apply, the songs from your selection sync to the iPod.  Going forward, changes to your iTunes library (based on your settings on the iPod's Music screen) sync automatically to the iPod, the next time it is connected (or when you click Sync).