Hey, please don't worry about posting. It can be hard when your therapist is away and you don't have that confidential and trusted person to talk with. It is great that you have been able to find another support in the interim online.
It sounds like using the drug has been a pretty big part of your life and that you feel like it has really helped you to cope with a lot of things - from what you've described, it would be incredibly difficult to completely stop using altogether. You are being really harsh on yourself - you are definitely not betraying your friend if you use again. All you can do is your best, and if she is understanding and supportive, hopefully she will be able to be there for you however you go.
Be open with her about how you are feeling and that you feel like it might be a bit too much too soon, that you would like to support her and keep her company but that you feel like you might not be able to complete the full 30 days. If you feel up to it, I'd really encourage you to reach out to one of the helplines available for substance use - they may be able to provide you with some advice and extra support, especially given your therapist is away. The Alcohol and Drug Foundation has a 24/7 advice line (1300 858 584), as does the National Alcohol and Other Drug Hotline (1800 250 015).
Please don't hesitate to reply on here, we're here to listen.
Take care.
