You sit down and you have a serious discussion. You tell her that you're not happy and you haven't been for a very long time. Tell her that it's gotten to the point that you are content to look elsewhere to have your needs met because this relationship isn't what you want. (This does not make you a bad person in any way) Tell her that you would like a divorce and that you've already gotten the advice of a lawyer (definitely do so before you talk to her about your final decision). She will be very upset. She will probably plead with you and ask you why. Be honest. The more honest you are the more she has to face the reality. Don't give her room to negotiate either because she'll probably try to. Then go out for a while and leave her some time to think and compose herself. Maybe even spend the night at a buddy's place or a hotel so that she can really have some solo time and you won't have her badgering you all night. At this point, it needs to be a very business-like decision. You don't have to be cold or cruel, but you do have to be firm.
When my ex and I broke up he did me the courtesy of leaving me alone as I packed my things. I had alone time so that I could scream and cry and I could quietly pack my things in peace.