Just browsing the forum and noticed this thread... Looks like you've changed to WordPress, but I just wanted to post an answer should anyone else need it.
Based on your description, the page title and navigation items are currently being generated from the same field (likely, the default "title" field) in the entry. Moreover, it's certainly not unusual to want them to be different (navigation title generally much shorter).
You'll want to add a new custom field specifically for the navigation title, adjust the template that generates the navigation to use this new field, and continue using the main Title field the way you wanted.
Assuming you're on ExpressionEngine 2.x (released july 2010), you'll add the new field through Admin > Channel Administration > Custom Fields.
To adjust the template that generates the navigation, you'll want to go to Design > Templates > Edit. If you're familiar with HTML, you can open the Index template and drill down to wherever the navigation is likely located at. You'll probably find an "embed" statement that references another template specifically for navigation. Just use whatever name is in the embed statement to find the navigation template, and go back to Design > Templates > Edit to open that template up. From there, you'll want to change the {title} tags to the name of the new navigation title field you created - {my_new_field} .
If you're on 1.x, the process is very similar, but the templates and custom fields are in a slightly different location - let me know if you need additional help.
I don't work for ExpressionEngine, but I do want to mention, for whatever it's worth, that I've used most major CMSs (WordPress, Joomla, Drupal etc.), and stopped when I got to EE. If you're comfortable with HTML, you'll find it's an extremely powerful CMS, especially for SEO.
Should anyone need any help with ExpressionEngine, you can find a list of developers at http://www.director-ee.com, or you can always visit their forums