What was your M like? Was it always up and down or was it happy before she got addicted to painkillers?
The reason I ask is that people usually dream of affairs/sex with other people when they are sub-consciously unhappy with something in their R, or sexually unhappy or maybe even just bored.
Your previous gf represents the new and exciting changes you wanted in your R and the sexual fantasies which you hadn't fulfilled properly with W.
Trying to cover up the affair would be you trying to juggle your own needs and fantasies against that of being a responsible H.
