Hi ETA - don't think I can help, but I can empathise completely.
I am in a very similar situation, and I know just how hard it is to get up the courage to do this.
My only words of wisdom would be to do it sooner rather than later. I can say quite catagorically that the longer you leave it the harder it becomes.
We have been having the " we can't stay together" arguments for years now, but never a proper grown up sit down discussion. Always in the heat of the moment. Our relationship is a complete sham, a farce, and I am a complete wimp and unable to sort it out. I have been plotting and planning for ages but haven't yet got the courage to do it!
I hope for your sake you find that courage from somewhere - sooner rather than later.
If you search through the posts on here you will see that we are not alone, far from it. There is a mountain of helpful hints and encouraging words to be had, all of which is very comforting - but it all comes down to you in the end I am afraid to say.
Good luck - will keep checking to see how you are getting on.