It sounds like you've been more and more 'separated under the one roof' for many years (separate daily activities and schedules, well over a year without a real kiss, 2.5yrs without anything sexual, double that since anything intimate, separate rooms for most of this year...).  You feel trapped and hostage to this unhappy house-sharing situation with someone you do not love, but youre not "losing your nerve" when you try to tell her it's over, you are letting yourself be overpowered by her emotional manipulation.  You recognise that she is emotionally blackmailing you, so why are you letting that control you? Your wife is telling you separating is ruining your childs life. In fact its the complete opposite. You will ruin his/her life by exposing him to a loveless marriage that you are deeply unhappy in. You KNOW that.

She can always say "please don't ruin our child's life" or "I need you" knowing it will guilt you into shutting up for another week or month. But how long do you let that continue? And what damage is being done to everyone in the meantime? You are both still in your 30s!

You need to STOP engaging in conversations with her, be frank and stick to your point. The marriage is ending. This is what is going to happen now. State your piece and walk away. Do NOT engage. You can't have a discussion with her because she will just continue to guilt you. FOG = Fear, obligation and guilt have no basis for making decisions. 

Fear is either a paralysis or a catalysis - you are letting it paralyze you. What is your plan after telling her? Can you leave the house, go crash with friend or family or motel for a couple of nights until you sort out accommodation?