I don't think you're a doormat. As crazy as it sounds, if you read around here, you won't find many husbands saying what yours is saying. I believe him - he got himself into a situation and he's too weak and brain-addled to man up and break up with her. Or he's doing drugs with her.
Either way, you haven't got much to lose by giving him a little more time. He's sick - you know that. This isn't about you.
