Purely my opinion, but I believe it's time to end the marriage. I've cheated in the past, my reasoning for it was completely different, but I still did it so I'm not going to judge you on that. If you're unhappy in your marriage then for you're own mental health I'd say end it. It'll feel like it's the worst thing you've ever done, but it wont be as bad as if she would catch you cheating. She may already assume you are considering you've already discussed your desires. She will be mad, hurt, and probably hate you, but if you do it sooner rather than later you might both be able to move on easier.
A lot of experts say that cheating could actually make a relationship stronger because it'll make you appreciate what you have. If it still isnt working for you then it's time to call it quits.