Okay I'm going to answer the OP with all the information out so far. Here's how we started with poly. I was suffering major depression. I've been a depressive for over 20 years, most of my life, and this was a pretty bad spot. Worse, a therapist sent from work to check on me agreed that 'whatever you have to do is fine.' Oh yeah, that did NOT go over well. It was basically giving me permission to do nothing but sit online and start an emotional affair. So yeah, started out cheating.
It came out and hubby was INCREDIBLY understanding even being as hurt as he was. When things went bad he actually tried to help us talk through things! That is so beyond fair I can't even explain.
It died, of course it did, it was an affair, it was all about secrets and lies. My second relationship wasn't a whole lot better. I wanted to be honest and up front but being scared I was basically lying to myself. No no, it's not love it's just, really good friends! Well, lying to yourself is lying to everyone else too.
The relationship I'm in now, is good. We all started out open and honest and transparent.
So the reason I'm going with all this little synopsis? I don't want to beat up the OP as someone with no moral compass and no right to be upset. I have been there and have screwed up big, and yes depression was part of that reason but what I had to learn was it was NOT AN EXCUSE. You can not say because you are or were depressed you made bad decisions and that's the end of it.
Forget getting to compersion, so not your priority right now. Get to a better place. Admit your wrong doing, work to repair the damage done by the lying and the secrets. Own your shizz! It's not going to be fun, no lie, it's not going to be easy, but it's worth it. We are now years from the affair, the emotional one, and there are still things to work through but we are in a better place and MOST of that is because I can own what I did, work to make amends and now work to prove I am trustworthy again. That meant with this new relationship, I kept hubby informed from the get go. We are talking, this is what we talk about, it's gone to this level, hey he asked about an actual relationship, told him I would think on it. Okay we are at the I Love You stage. Hey there's cyber sex going on.
So yes, it may seem unfair that you have to go through all of that and he doesn't, but YOU had the affair, so YOU have to repair the damage done to the foundation of your marriage. Build it up again and get to a better place. It's possible. If compersion is that important to you, look at where hubby and his gf are and change your thought process. If you've suffered depression for long I'm sure you heard about the cognitive behavior thing. How you think, how you act, how you feel. Change one. So look at them and change how you think. To change the resentment change the thoughts. Try, "Wow, I hope I can work my way into having a relationship that open and honest." See it as a goal, not something you CAN'T have, or was taken from you, because it wasn't. See it instead as what you are working towards by working on your marriage and learning to regain trust.
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year