I had same problem Kiloco. Nobody knew about my xabf problem (he's coke addict). I felt stupid as we've been together almost all the time. He hide it so well that neighter his family, coworkers, friends, best mates and flatmates new! Literally nobody! I was shocked as well when he told me, I actually though he was having a laught...
Well we've got right not to know. I never knew Anybody who was addicted, i didn't know symptoms. And as you me and my x been together for 7months. It's the time when you learning the other person and build your trust. It's easy to be manipulated as you don't know him well. Being tired and moody - hard day at work, fine. Being secretive - I like to solve my problems by myself, fine. You just believe in every single excuse.
But it's a good question, I ask it myself everyday: how to trust again? No clue...