Hmm, in my case, I tried not to tell. I tried to distance myself without him noticing. But he knew it almost instantly. He felt me withdraw from the role of enabler and he was mad. He made me fess up and as horrible as I thought it would be, it wasn't that bad. I had already had the imaginary discussion in my head so the details just flowed out. After being sad, he somehow decided that we were leaving each other, instead of me leaving him. Before I knew it, we were discussing how best to handle it with regards to the kids, how to divide the furniture and moving dates.
I don't know your situation, so only you can judge best. Are you in danger? If so, a contigency plan is in order.
Perhaps you need to consult a lawyer and talk about your options before you tell your husband. Have your ducks in a row mentally and then perhaps see how you feel about telling him. I can't imagine having to engage in "pretend sex" can feel very good.
I know you want this to be pain-free, but the reality is that it almost never is. It's the end of a way of life, of an idea, and of many toxic patterns. There's bound to be grief, anger, sadness, emptines. But all that discomfort doesn't kill you; you live through it. It just sucks the big one, for all involved.
good luck...we're here for you!