Not saying she is definitely into anyone else. Just see if she is keeping her phone close to her at all times, doesnt answer the phone sometimes when it rings or leaves the room, takes off sound indicating message arrived, changes password or does not let you have access to her phone.
If this is the case she is most likely involved in an EA, in other words is texting sexy, lovey, emotional and personal things to someone else. If that is the case you are a step from having a W getting into a PA. She would also at this point be classified as a WW.
Read up on what an EA and PA is. No one ever thinks their wife will do such a thing.
Also a few other points:
1) You cannot repair your M. It is over the moment she wants out. 2) Do not go into explanations about whether it will be the first or last fight 3) The reason she is worse than you or more hurtful is because she is not holding back. She does not believe in you or the R so there is no reason to pull punches. That is why there is no sense fighting 4) I also think it is important you find out if there is an EA or PA because if there is then there is no point going to a MC.
On a final point. Stop thinking about her and start walking away and doing your own thing. She needs to see you are walking away and not focused on her. Do not get involved in arguments, walk away but never if she is disrespecting.you.
Stop her and then walk away. Always try and leave with the last word.