So many of the comments in this show you why we have so many fractious families.
Snooping through your phone was wrong, but it can't have been nice to have read awful messages about themselves.
I would say the best way to move forward is to have a frank discussion about privacy and respecting people's right to it. Also discuss the things you've said, and why you've felt the need to vent to a friend.
Then draw a line under the experience and move on.
All the suggestions for kicking a 19 year old out, or stop helping financially, or cutting off WiFi are ridiculous. They wouldn't get any housing if you did kick them out so would be homeless, and at best in emergency accommodation such as a hostel or similar. Times are different now, very few people are financially able to live out at 19 even to rent a room you can pay upwards of £600+ a month.
It's our jobs as parents to help out children to be in the best position they can be, not throw them out because they're 19 and not respecting our rules.
Defiant and difficult behaviour between 18-25 is common, ride the storm as best you can and remember that you're also not perfect.