Sorry man... that situation just sucks... but many like yourself have been there and are still live and doing great. I had a similar situation with my first wife yet it did not even involve substance abuse. Go seek counseling... do it for you and your son. You wife has moved on, and sometimes it is easier for them because they already had a lover during their addiction (her's was call heroine). 10 days in rehab is nothing, and there is no surprise she relapsed.
You need to take care of yourself... let the past go (easier said than done) and move forward. Learn from the pain, and the experience... make yourself a better person.