Hi losingDD - I wish more parents were like you !! - I have worked in substance misuse for 10 years. You really can't make her do anything that she doesn't want to do - but you can support her and be well informed. Most areas have courses for parents run by the local DAAT (drug and alcohol action team) - it's a national requirement. They can be very informative - around drugs, their effect and how to deal with the fallout. Also useful for meeting other parents in the same situation. Let me know what area you are in.
Also - in my experience - young people find it really hard to go to their GP's or drug services to get help - it takes a huge amount of confidence - even as an adult. I have had several of my clients complete an online help course specifically relating to cannabis. Link here
www.knowcannabis.org.uk/selfhelp.htm
a bit more anonymous.
Often cutting down before stopping is more realistic - ie. smoke only 2 joints a day or every other day. Although not physically addictive, cannabis can be hugely psychologically addictive, especially when people are using it to block out problems.
Hope this helps