Mountain Lion email share option opens a secondary Mail application
The most likely explanation for this is that you have at least two versions of Mail installed (for whatever reason). Your system default mail application happens to be the old version.

When an application uses the sharing API on ML to write an email, it is opened in the default system mail application. This setting, strangely enough, is inside Mail's preferences (not System Preferences).

Under the General tab is an item called Default mail reader, which is your system default.

In your preferred version of Mail, Open Mail -> About Mail, and note the version. Now select that version of Mail in Mail's preferences for Default email reader.