His drinking is not a reflection on you. It's a reflection on him. Alcoholics protect the parameters of their addiction. He keeps you at arms' length so you don't ask questions and make demands that would change his routine.
What would happen if you said to him, "Hey husband, I know you're out in the shed drinking every night, and I know you're hiding it from me. What's up with that? It's weird and it worries me." Not as a fact-finding mission, but as a way to let him know you know. To break that ice. To make it not a secret anymore.
You sound like you may be isolated. Google Al-Anon meetings in your area and try to get to one. It's to meet people and to talk to others about how you feel, how the alcoholism has affected you, and to learn new ways to cope, so you can sleep through the urge to search sheds for answers.