I cheated in my last long term relationship.
I'm not proud of it. But I also don't feel much shame. Perhaps I should. And I did at first.
But now I more than anything feel resentment towards my ex (yes, it ended the relationship).
It is not fair, but the reality is that our impression of others has more to do with how they act towards us than whether or not how they act is justified.
In this case, she reacted with extreme anger and resentment. At first, I was apologetic, and genuinely felt sorry and upset.
But after a while I came to accept the situation: The relationship was over. In many ways it was for the best.
And I started feeling that I had paid. I understood and understand that she still didn't trust me, and was angry, but a shift was occurring: When she had an outburst of anger, it started feeling excessive; unwarranted; unfair.
Especially when the attacks were - to me - inappropriate or in a form I'd never consider myself: When she broke plates. When she threatened me (though I know she'd never carry out any threats); when she hit me (I'm so much stronger I never considered it a physical attack - what hurt me was the anger behind it).
Each time, I moved one step further away from sympathy and feeling sorry about what I had done, to rationalizing my behaviour and writing off her response: If her response is irrationally strong, then it only figures that what I did was not as bad as I originally thought, right?
Of course this is nonsense, but it is how we tend to react. What I did has not become any better. I may feel better about it, but her irrational and extreme responses show just how horrible it made her feel. My actions have not become morally any more (or less) defensible. I feel responsible for her anger. But despite knowing and feeling responsible, I still have a hard time mustering the sympathy I ought to feel due to how excessive her response feels to me.
We are amazing at protecting ourselves; our egos; against truly taking in the consequences of our actions. Whether or not someone elses response is rational or not is largely irrelevant to how we react. What matters is that it takes a lot for us to accept criticism or reactions that does not mesh with our self image. I don't see myself as an evil, immoral person, so when my ex calls me evil or immoral for "just" cheating, my natural response is to label her irrational and over the top. I can logically understand that she responds this way out of anger, and that I should have known logically how she felt about being lied to, but that does not change my emotional response.
Think about that the next time someone does something "unreasonable" to you in response to something you said or did. Even if you don't feel all that sorry, consider that the way they act is a direct result of the fact they come from a completely different viewpoint, and chances are that you may alter your view of them in ways that might not be justified (my ex is in most ways one of the kindest, most generous people I have ever met, and in the midst of my resentment I still daily see examples of this, such as when she repeatedly as the only person on a packed bus will pay attention to when someone needs help or need a seat and ensure they get it, even if it means cajoling grumpy commuters that would rather pretend they can't see or hear, yet the kindness I see does not wipe away the resentment I feel).
Think about that whenever you consider your responses to something you consider wildly unfair. Attack back, and the person you attack will not suddenly agree with you, they will come to resent you more, and reconciliation will become harder, not easier. You may in a single blow wipe away ten times as many incidents where you are kind and helpful and do everything right.
It is not a given that there is a "right" way out of abusive situations. If you can remove yourself from the situation, do (with my ex, what stops me from avoiding her is that she is the mother of my son). If you can't, to man up and take the abuse and walk away without responding is often the best approach if the emotions behind the abuse are valid. I accept and tolerate the way my ex attacks me because no matter how her continued attacks make me resent her, I accept that it is genuine anger of my own making.
But whether the emotions are warranged or not, you need to consider whether it is worth it vs. the damage standing up for yourself can do. An abusive relationship is not worth it. Stand up for yourself unless there are clear, concrete reasons to tolerate the situation (such as children - but even then there are situations you must address, such as if she gets abusive to you in front of them, or tries to sabotage your relationship with them). The resentment that will build if unchallenged is toxic, and you need strong reasons before you allow it to fester.