I feel like there should be some free counseling services available through your local cancer organizations. Also, consider a support group - maybe that would help?
And, to be honest, there's a lot to be said about appropriate use of prescription drugs. I use XANAX to help me sleep at night when the bad voices are active in the back of my head (to use your language - I usually just call those the "what if" thoughts).
Lastly - consider getting OUT with your grand kids. Take them fun places! Let their joy overwhelm your anxiety....
Hugs. I'm sorry you are dealing with this.