It's so simple you would have to be void of any cranial goo to stuff it up.
Step 1: Put nicotine patches on them while they sleep, they'll crave you and be addicted to you. Just remember to remove them before she wakes up.
Step 2: Now that she is addicted to you. You have to cement you place in her life. Do this by drilling it into them they are useless, ugly and you are doing them a favour by staying with them. Bonus points if you get them on antidepressants.
Step 3: Enjoy your life partner, you should be able to cheat on her all you want without her do anything about it.
Step 1: Put nicotine patches on them while they sleep, they'll crave you and be addicted to you. Just remember to remove them before she wakes up.
Step 2: Now that she is addicted to you. You have to cement you place in her life. Do this by drilling it into them they are useless, ugly and you are doing them a favour by staying with them. Bonus points if you get them on antidepressants.
Step 3: Enjoy your life partner, you should be able to cheat on her all you want without her do anything about it.