Why would you use diary when there are perfectly good open source password managers like KeePass?
Trusting your brain memory to remember all your passwords, or using one-for-all password is worst thing anyone can do.
Note that good password should ideally be long and randomly generated, that is not something you can remember or write easily.
A strong password must be unique like you said and complex enough as well. Because it is complex, it's impossible to remember bunches of strong and unique passwords by your memory which can be lost by an accident or by a long time does not type the password manually.
Below is a guide for OP.