Jenny,
This may be a little controversial here and if so I am happy to remove the post or for it to be removed.
There is many worse things that your daughter can do in life than smoke a cigarette.
From personal experience those that are going to try one will do so regardless of what they see around them.
Before I ever started smoking my grandad lost first one, then the other leg to smoking related disease. Sadly it took his dignity to such an extent, that he didn't want his family around at the end.
When I had been smoking just a brief time I lost my Nan to smoking related illness. Officially pneumonia got her.
When I was in my early 20s I lost my dad to a massive heart attack brought about my smoking. He was no more than 4 years older than I am now.
In my thirties my mum had to have major surgery due to a number of years smoking.
The point of this? We are each our own person and those choices we make are our own. I could justify it because I would never allow myself to get that bad, it wouldn't happen to me.
I know from personal experience and the experiences of thousands of others here that nicotine addiction is so surreptitious that nobody recognises or cares to recognise the point at which it takes control. This we need to teach people.
It wasn't until I recognised behaviours I didn't like in myself I decided to change. Then I was ready to be truly educated. It was only at that point I could really understand.
The things I emphasised to my kids were to be kind and not wilfully hurt others, anything else we can deal with. We have had some interesting discussions and I'm happy that they do speak to me.
It may sound flippant, but try not to regret the past, it serves little purpose. What you should do is rejoice!
Your daughter has grown into a young woman who will make her own choices, for better or worse.
She is a young woman who is brave enough to talk openly about experiences.
And most of all that she is a young woman who trusts, loves and respects you enough to speak to you about it.
You sound like you have a great relationship, try not to be too hard on her or yourself.