I think you need a new nanny. Sounds like none of you are especially happy. It's ok to move on - things were great for a while but now you need a different configuration for your family and maybe she's ready for something new also.
You can work on a replacement, give her a great severance when you have someone lined up, and move on without guilt.
Good luck!
Why on earth would anyone give a nanny like this a great severance package???? OP search for someone new and follow the notice period in your contract. Please do not pass her along to another unsuspecting family with a great, false reference.