Dangerous ground, that.
My mom went through the same thing from when I was 13 to when I was 18- over a guy she works with. And it too, was one-sided.
And here's the thing- she told my dad how she felt about everything. And she had no friends to talk to and so she told me everything too.
BIG MISTAKE.
It was very detrimental to us as a family. I was already dealing with depression and suicide and that all made it many times worse. And my dad was depressed and suicidal.. and she went to a counselor once (and the person wasnt a good fit) and that was that.
We just had to suffer through it.
Some suggestions?
PRAY.
Stop emailing her. It may be one sided, but its still affecting you and indirectly your marriage. Continuing it - even though it seems just harmless on the surface - will indulge your little in-your-head thing, involve this other woman in something she has no idea she is being dragged into (and consequently people will hate her.. your wife.. children.. because of YOU when she has done nothing wrong and its all your fault for allowing yourself to continue this)
Don't lie to your wife. Don't necessarily offer info, but if she asks, be honest.
If you have kids, DONT let them know what is going on, no matter what.
FIND A MARRIAGE COUNSELOR. At least go by yourself. And if your wife knows, you need to go together. At some point in the counselling, you should go together anyway.
You need to nip this NOW. Letting it go will just cause more pain and hurt down the road. Instead of investing thought and emotion in the people who deserve it (your family) you are thinking of her. And even though you are not having sex or anything, sometimes emotional investments are stronger than physical ones. And that means more detrimental, too.
But stop ALL contact with her- explain it how you like to her but her being hurt about it doesn't matter when your family is hanging in the balance. Find a counselor NOW and go, either alone or with your wife, but GO. And DONT tell your children about this.
I don't have time now to go into more detail, but if you want to know exactly how my mother's doing this affected my family - for ten years now and still affects it - I will be happy to tell you.
But let me just make it clear, you are on dangerous ground. And I see that you say you want to get over it, but I hope you are sincere in that desire and not just trying to put up a good front. Because if you are not sincere about it, it will end up compounding the pain that will result. I pray that you are sincere and that you will honestly seek counselling and do all you can to stop this. For you and even more so for your family.
But if you are sincere, which I hope you are, find a counselor today. The longer you put it off, the longer it goes and the worse it gets. And if the counsellor you find isnt right, try another one and another one and another one. Until you find one that works for you. Your family needs to come above and inconvenience this process may be to you. You started this in your head and its up to you to finish it.