Hi Sketscher and welcome.
When I first saw the title of your thread, I thought you were the woman my husband had a relationship with. Don't worry, I'm not here to judge you. You are hurting just like the rest of us and no one is perfect. You deserve compassion and support just like anyone else.
I told my husband's friend about al-anon and I hope you go to a meeting. No one deserves to live with the misery of active addiction. You can't save him, he can only get well with the help of a higher power and a recovery program. But you can get help for yourself and the first thing I hope you do is forgive yourself and move past the guilt.
Take care,
JG