Methadone has helped may people on this board. I don't know how long you have read the board but there have been many god discussions on this medication. You might want to do an advanced search on methadone to find them.
Good luck and I pray that you find some relief from the pain. I had a hysterectomy at 24 years of age for stage 4 endometroisis, adhesions, and hemorraging so I can understand how painful it is.
I have taken muscle relaxers in addition to my pain meds for endo pain. They can help by relaxing the uterine contractions, and my relaxing you in general. Some work better than others. I have tried Robaxin, Soma, and Flexeril with varying results. The Flexeril simply helps me sleep....good on it's own. The robaxin and the soma have helped me added to the pain meds.
What are your doctors doing to treat your endo? What has been tried so far?