Hi Sarah,
This is a tricky situation for you to be in.
I do feel for you and understand this can be very stressful for you.
It it helps in anyway, the one thing I love that my mother taught me was if I make a 'mess' I need to clean it up with all involved. Taking responsibility for my actions. I totally understand that this is a really hard thing to do in this instance, (it has been for me also in many instances I've had in my lifetime) but ultimately, it's been the right thing to do and it removes guilt and done right - potentially regret as well.
There are always consequences to our actions/choices and this is where we learn and become adults in our behaviour and response. This will be painful for your fiance as well.
Brene Brown is an amazing author on vulnerability and relationships these video's may be of help:
https://www.youtube.com/watch?v=psN1DORYYV0&t=1shttps://www.youtube.com/watch?v=iCvmsMzlF7ohttps://www.youtube.com/watch?v=bf5zCV8xEPwhttps://www.youtube.com/watch?v=8-JXOnFOXQk&list=PL6StD5HhGmVVM4Goruc-rFusfG140VJ-xYou can do this and you will get through, it will hurt, and you can do this well.
take care
xx