I always go back to feeling this way. I even look at pictures of him during the A, and he looks so distant but with a smirk on his face like "I know something you don't know", or "I'm fantasizing about her right now". The thought of him getting aroused by thoughts of this woman and the context of their texts and phone conversations is just appalling. I, too, imagine having the "fun" that he got to have.
I especially feel it is unfair because I had only been with one other person before getting married, while my WH (who is 12 years older) had already been with like 30 women. So, really?? Didn't he have enough experiences to get a good idea of what's out there? He really had to go find out once more from someone EVEN YOUNGER than me??
But then again, it's about their insecurities, not about you. It has NOTHING to do with you. I feel just like you - I am the breadwinner and kept myself in great shape for him. While he was a stay-at-home dad and I was proud of it, now I just get angry about it because he put the kids in front of the TV so he could go have phone sex!! While I was in the office paying our mortgage.
None of it is fair. But like our MC says, life isn't fair. Getting cancer isn't fair. Losing a child isn't fair. Being betrayed by your spouse is not fair. But we will come out of this a deeper, stronger, richer person. I have to believe that!