Baruch,
I use to remember what it was like to have babysitters in our house. Every time a new one was due to come, I would decide that I would NOT even look at her to see if she was attractive etc. I was NEVER successful!
Here is my advice to you:
I personally know of a well known Torah family, (of fame), and they did the "chessed", of allowing their young Bais Yakov daughter, to help out with another family, (also of fame), while the mother was out of commision and not in the house, the father was given assistance each evening/day (I don't know all of the details), but I knew both families, and after some time had passed, I was curious why the father divorced his wife, and then married this very young Bais Yakov girl, (about 18 years younger than him.) And, I couldn't understand why this respected family would have their daughter marry this older man with several children.
Well.....while the young girl was helping out in their house, and the mother was not around, he couldn't control himself with this young girl, and....he got her pregnant! So, the wife comes home, finds out their babysitter is pregnant by her loving husband of many years, which was of course very painful and hurtful to this women. I will leave out some other details, but in the end he divorced this women, and married the young pregnant babysitter. Both families went through tremendous embarassment, the home of one family was broken up, the kids were taken away. There were court hearings and terrible battles on who would get the kids. Some of the children were tought to hate the "other" parent etc, and this young Bais Yakov girl, who had her entire life ahead of her, in a moment of lust, lost her zivug, lost a beautiful wedding with the picture album to last a lifetime, and was forced to marry a man much older than her. Do you get the picture?
Now, there is no way you can continue in this manner unless you are willing to go through this scenario above.
Here is my advice:
The Baal Shomer Emunim zt"l wrote in his sefer Taharas Hakodesh, that as soon as an improper thought enters your mind, you must immediately push it out, "Im Shnei HaYadayim", (with BOTH hands). i.e. one hand is not enough. He has to be eliminated immediately. Each mili second that he is allowed to remain in our minds, is TOO long! So, we see that the battle with the yetzer begins in the very beginning. At the start. When we have allowed him to already exist within in us, it is already too late. As we here all see. If we don't beat him in the beginning, when he starts up with us, then we will not be able to in the end for he is already much too strong for us. Each moment he gains more and more strength.
You must either tell your wife that "we need to get a new babysitter", and the embarassemnt of telling your wife will be an incredible tikkun for you. Of cours if your wife will go crazy when she hears about this, then this is not an option. So, you have no choice, (unless you want to turn out like the story above), other than to leave the house everytime the babysitter is due to come. You will have to find somewhere to go, to learn or whatever it is you are doing. You must get out of the house. You have made yourself a case similiar to one leaving his computer on with porn on the screen. Will he never glance at it? Impossible. You must take the advice of the Baal Shomer Emunim, and start this battle "from the beginning". Get it back to the beginning stage, and leave the battle field, i.e your house. Obviously there will be no more email writing etc.
When I think about this man, I feel bad for him. I can't blame him because I know that if I were in his position, I might not have done any better. But, when I go over the details, and all of the pain...we all know that it was never worth it. But, still the yetzer has the power to blind us from these details. Therefore, our only true chance lies when we beat him at the start, before we get so deep into the tayvah.