Sadly, your story has a familiar ring as it is all too common for active addicts to lie, manipulate, try to control their enablers and build hope then dash dreams...all because of drugs.
I think you already realize that there is nothing you can do to change her, that will come (or not come) when the pain of using becomes greater than the fear of living sober. My son, the addict in my life, has been on this drug journey for most of his adult life, leaving women and children in his trail and he goes from active addiction to rehab and recovery and then relapse and the cycle continues, getting worse each time. As I write this he has recently been sent to prison for some major drug dealing and car theft. I have been distanced from him for about 15 years now, I just can no longer live the life of someone who loves an addict, no matter what the relationship. I pray for my son each morning and let God take over for the rest of each day. I live well today, a happy and healthy life of healing and learning each day.
The fact that she is on a methadone program brings promise but is no guarantee that she won't relapse again. I pray for her sake and for that of her daughter that she can find and stay on a good path.
Your part in all this? Mostly it would be taking care of you first. If you can drive her each day and help her to help herself without getting emotionall entangled in all that addiction brings, then good for you. But be careful, you wouldn't be the first to lose your heart (and possessions) to someone living with addiction.
For me, meetings helped. Al-anon, Nar-anon and CoDA are similar fellowships that are about US and not them or their substance choice. I learned to set boundaries, I learned to decide what I could live with and what must change if I was to live a healthy life. Meetings and Soberrecovery helped me surround myself with support...for me...and saved me from myself and my codependent ways.
Welcome, I hope you stick around and join us, you are not alone on this journey. There is some good reading on the Sticky threads at the top of this forum. Maybe take some time and read and learn from those of us who have been there and found a better way to live.