As hard as this is to hear, my life learning experiences tell me:
You cannot control anyone else's feelings, thoughts and actions. You need to let your mom go and do what she thinks is best for herself. If you have love and compassion left for her, you can catch and support her when/if she falls.
Some people think the grass is always greener and they can always do better with relationships. Hopefully one day they will realize the way to live life is appreciate what you have and build on it. Good things for good people and the karma continues. Honesty with yourself will set you free.
Your mom is not telling you the entire truth. She will hide behind "I am keeping the truth from you to protect you" BS. Learn from this so you do not repeat this in your own life.
I wish you well.