BH Cee64D - 50
First of all, welcome to SI and sorry you find yourself here.
Second, you did good by coming clean. Not many WS do that without fighting every step of the way.
Third, there are a few hard and fast rules if you really, truly wish to help your BS heal and save your M. The first is complete honesty. You answer her questions. Every time. No matter how many times she asks. And you answer truthfully even if it makes you feel and look like sh*t. She deserves nothing less.
You be an open book. No hiding *anything*. You want to rebuild trust? Then you give her reasons to trust you. This will take a long time and it's your actions that will count. Right now your words are meaningless. She has no reason to believe anything you say right now.
Find out your why. Why you thought having an A was a good idea. How you justified it to yourself. Why those "reasons" made it okay to tear your BSs world apart. Not easy. You'll have to take off the blinders and take a good, hard look at youtself and face things about yourself that you probably won't like. This is necessary so you don't make the same F'ed up choice again in the future.
R isn't for sissies. It's hard work and you'll have to do most of the heavy lifting. There will be no instant results. This is a marathon, not a sprint.
You'll find great advice here, the support you need to get through this and the occasional 2X4 if we think you need it. To stay on the right track. We swing those to help, not to hurt.
Keep posting, asking questions, telling your story. The more we know, the more we can help.
Again, welcome to SI, the best club no one wants to join.
FWW (me) - 51
All affairs are variations on a theme. No one has 'Beethoven's 5th' to everyone else's 'Chopsticks'.