If you are paranoid that someone could be reading all your private text messages that you sent to your girlfriend, then you need an app that can provide you secure and private channels for those late night text communications. Fortunately, there are many apps for both Android and iOS that allow secure encrypted communications. Signal, for example, is an app available on Google Play store and Apple App store that can provide you with very strong secure communications.
Signal is a cross-platform app and works on all the popular platforms such as Android, iOS, Windows, Linux and Mac. Endorsed by many well known journalists, this app uses your existing mobile phone number to identify you and does not need any other way of creating accounts etc. This makes it very easy for anyone to start using this app without going through the usual hassle of creating accounts.
It can access your existing address book and contacts so that you can start using this app right away, select a contact and send them a text message. Furthermore, you can create group chats where you can invite many of your contacts and send texts that reach everyone in the group. Of course, all of these people must have Signal installed in their phones for the group chat to work.
In the settings, you can choose whether to allow Signal to become your default SMS or MMS app, whether to receive the notifications from Signal on the lock screen, use keyboard app in the incognito mode (prevents it from learning about the words you type), relay calls through the Signal server to prevent others to see your real IP address, block taking screenshots of Signal messenger and more.
For ordinary users, there is no real reason to use a very secure app like Signal, but if your work requires privacy such as investigative journalism then you may like this app.
You can download Signal for Android from https://play.google.com/store/apps/details?id=org.thoughtcrime.securesms and for iOS from https://itunes.apple.com/us/app/signal-private-messenger/id874139669?mt=8.