You are doing the best you can, but please get R out of your head. Your wife has chosen to destroy her family and marriage and the catalyst is a "spinning instructor" . She regards you as the meal ticket.
When this first tarted, we all told you that the reason she would not leave the gym was because she wanted to try to keep fucking him, and that if his girlfriend ever broke up with him, your wife would bang him again in an instant. THAT IS STILL THE CASE, so what is the to think about R for.
You need to kick her out of your house, let her get an apartment quickly or she is going to be using your home as her sex romp room like she already has. She will find someone else if the spinning instructor does not get the urge to have her again.
You will do yourself a favor if you stop caring about why. She did it, she wants it this way, so what you should be doing is hiring the best lawyer you can afford and making her life as miserable as she made yours. make sure she has to go out an get a job. That will rock her world.
Please do not let your 13 year old melt you into getting back to where you were. She will adjust, Millions of kids turn out just fine from divorced situations, and you will be a much better father when your soul is not being destroyed by this idiot you are now still forced to call wife.