Euthanizing at home is not as easy as using human medications to do it. This can cause much pain and suffering for Sheehan.
What I suggest is to have a Housecall Vet come to your home and do this for you. If you would supply a City, State and Zipcode, I would be happy to try and locate one for you. If I cannot you may want to call your regular Vet and see if they would do this for you.
I know you want what is best for Sheehan and this is not an easy situation, but having someone come out and do this with the proper medication is very important for her to pass peacefully.
There is no Human Medication that will do this without unwanted side effects. Xanax can have a paradoxical effect and actually cause high agitation and anxiety which is supposed to relieve.
I wish I could give you an easy answer, but the best way is to see if your own vet would do this at home for a minimal charge or call out a Housecall Vet. I understand the financial situation, but the only way to do this without causing suffering is to have a Vet do this.