Hahahaha oh man
First of all, jailbreaking isn't hard and doesn't take long to do it, even though if you're not experienced
Second you don't need some fancy tweaks or apps to hide stuff, you can just use iFile, one of the most powerful apps in Cydia
Third; I saw/see a lot of tweaks and apps which hides or (password) protects stuff like messages, mails, pictures etc.
If you don't want someone to see stuff like mails, messages etc. either don't write them or just don't give anyone your phone. Put a pass code and that's it.
If someone doesn't trust you then it's the wrong partner to be really honest. If someone give you the feeling that he/she is cheating then you should just talk instead of checking the partner's phone :/