Password protecting an Application
It's usually left up to the writer of the application to make a password work the best. If you forget to close the app or if you put your mac to sleep, any wrapper you construct will likely not secure the diary. Building the password into the app makes it far superior to a gateway app that asks for the password and then launches another app.

That being said, have you investigated these nice journaling software that include the ability to password protect your files?


Mac Journal by Mariner Software 
Memories by Coding Robots
Diary by Michael Gobel


There are easily another 10 viable softwares on the App Store alone that serve this exact niche. Also - this review seems to cover some other options, but is about 12 months old now.