@ppeatfruit
Gee There has been research which shows that the brain is not fully developed till late 20s (or older) she is 19 but she is NOT an adult; teenagers are well known for pushing boundaries, sleeping late, being pains etc etc. it's part of growing up and in some cases it takes longer than in others. I'm not saying it's easy, it is just what it is.
I agree with this.
My DD didn't sort of start to act like a grown up IYSWIM til maybe 22-23. Even at uni she was a little bit flaky and irresponsible, and a little bit flighty. I mean, she managed her academic studies and went to her part time job (sometimes!) but she was a bit flaky at times.
She would think nothing of asking us to pick her up from the station at 5pm (fair enough there is no bus to our house,) but it was 30 to 35 minutes drive to the station, (at the time.) A few times she missed her train, and forgot to let us know she had got one 40 minutes later. So we'd be sitting there at 5pm, and she didn't get off the train.
So we'd ring her and say 'where ARE you?' 'Oh!' she would say 'I missed the 3.30pm train had to get the 4.10pm one. So we'd sit there three quarters of an hour waiting for her! It wasn't worth going home, so we wasted our time. (I know we should have told her to get herself home! But we could never bring ourselves to do it.) 
She also forgot our birthdays for a couple of years in a row AND one mother's day. I know it sounds like a first world problem, but we found it hurtful.
She would also ask us to come pick her up from her friend's house (when she was 14-15,) at say, 4pm. So we would go up to the town at 2.30pm as we only needed an hour and a half to do what we needed to do, and she would text (at 3.40pm) and say 'can you get me at 5.30pm instead?' So we were done - with what we needed to do - by 4pm as we planned, but had to hang round til 5.30pm as she moved the goalposts/changed her mind!
Again, it wasn't worth us going home, as it would have taken 25 minutes - then 25 minutes back! So we'd have got home, and just had 35-40 minutes sit down, and then have to go back out to get her!
I know. MUGS we were. But after the first 4 or 5 times, we started saying 'NO, you said 4pm, and that is the time we have tailored our time out for, so it's 4pm, OR your friend's parents can bring you home.' She reluctantly said 'all right then! I will be ready for 4!' 
And as I said, her flakiness and pissing us about (sometimes) carried on into her college and uni days.
But by 22-23, she was like a different person. Totally thoughtful, kind, generous, reliable, dependable, hardworking, empathic, and great fun to be around. 