As Michel states in one of her videos, any R where emotional needs are not being met within the R, is at risk of infidelity.
Your fears are justified and understandable.You have two options as I see it, but you can do both. In your shoes I would want to rule out or confirm the fear by snooping. If you find nothing it reduces your fear. If you find something I think that could be good for you too. Yes it'll send you in a spin, but all the guilt and self blame and martyrdom you have endured during this saga can finally be put aside. Do not snoop if it is against your convictions and don't get caught. This is not about controlling W, but about helping you.
The second thing you should do is use CBT to deal with your issues and anxiety.I recommend the excel at life website. They have audios and transcripts of audios so you can either listen or read. Or use whatever techniques you have gone through in IC.
A third thing you could do is make the most of the time she is gone. Her presence weighs on you. Her not being there will creat a lighter atmosphere.At this stage you should already be doing whatever you want to do, but plan something extra special for when she is gone.
Best wishes
R 25 years M 14 years S11 & S13 Working on it alone since Oct 2014 M in trouble a lot earlier (~2 years) Feb 2016. 1st R chat in a yr. Next R chat Aug'17 Still together