I felt lots of guilt the first time I cheated. Swore I never would do it again. My wife forgave me and I cheated 3 more times that year knowing that she never once mentioned a divorce. After that she decided that I was not going to change from the poly person she knew I was long before we even met, so she joined me in some groups sex that led to her falling in love with her best girlfriend who was part of our marriage for 30 of its 44 years.
I know the guilt thing and nothing more emboldened a cheater than knowing that his or her spouse is afraid to divorce or leave them. The married women I dated had been all caught before, went through the guilt trip but, when you see that it has no serious consequences, the guilt goes away. I have been cheated on and cheated. I have been poly for most of my adult life too. So I know the unnatural chains of monogamy and the freedom of poly. I know how it feels to cheat and how it feels to be cheated on.
It does get easier the next time. The problem is that a person's past behavior is a pretty good indicator of their future behavior. There was a good article on the CNN website this week. It is amazing how so many people accept a form of relationship that fails 50% of the time rather than try something that may save the marriage. You can read the article here: Rethinking monogamy today - CNN.com
The problem is that it takes an very long time to regain trust and even when you do, it will never be the same as it was at the beginning. You will be forever suspicious of him, bring it up in arguments and he will grow to resent it over time. There are no winners when one partner cheats. I was lucky in that my wife did not want to be monogamous either. She was a virgin and curious about sex with others. Then she started to have a desire for women as I already did I. We both thought that it was crazy to try to live in a marriage structured many thousands of years ago, in a time when the reasons for monogamy had a real purpose. So we restructured our marriage to fit us and it thrived for 44 years. It should be noted that between us there were less than 10 outside lovers, with 4 in the second year of marriage, over our 44 years together. That is less than many married people I know who cheat have, in just a few years of marriage.
Not trying to sell you on my lifestyle because you need the right mindset and emotional control to make it work. I just cannot understand why so many rather go down with the ship rather then find another ship to stay afloat on. Sex with another during a marriage is only the terrible sin it is perceived as because we say so. No matter how advanced we think we are, we still cling to some aspects of marriage being an ownership of each other or at least the ownership of physical pleasure of each other.
Hope you work it out. I never loved my wife less when I was with another women and the same goes for my wife. We always held each other and our marriage above all else and it worked for us. What I do not like about cheating is the lying and deceiving that it forces you to engage in. The sex part does not bother me as I never felt that my wife's sexual pleasure belonged to me. It is hers to do with as she wishes, as long as I have her heart.