iOS 12 ScreenTime & New Parental Controls
A huge focus of iOS 12 is on controlling how often you use your iPhone and adding in new controls you can use to keep your behavior in check as well as a massive upgrade to Parental Controls for the iPhone an iPad.
ScreenTime will let you set limits on how long you can use an app each day and then warn you as you get close to that limit. It's a great way to keep yourself from spending too much time on Facebook or Instagram.
You can also set these for kids that are part of your family and even link this to a ScreenTime Allowance that you can set to keep limits in place and to add to them when kids do something. With this feature you can limit how much a kid can use Fortnite, but leave other apps for homework and communication wide open.
This syncs to mac and you get reports on your own usage as well as kids usage.