Sleeping with my ex-husband, do I break my silence?

Desperately seeking your advice! Background... I was married for 13 years/together for 20 when I caught my now ex husband cheating with the girlfriend who he is still with. (Note: she knew he was married at the time they started talking). Our marriage was over as much as I wanted to try for our son he wanted the girlfriend.
Watching the pain our son went through was almost to much for a mother to bare & to this day he still hurts from the split.
I still carry around so much guilt for the pain we caused him!
Let me be clear, I do not want my ex back! He is not a good father or significant other! That being said... we “got together”. And we have been ever since, going on 4 years now. We’ve tried to stop several times.
I am dying of guilt and need to release it!
Even after the part his girlfriend played in helping to end our marriage she doesn’t deserve this! She really is a good person & is truly great with our son, probably the best I could ask for! She deserves better than my ex!!
I see a lot of me in her thinking he would change, was hurt over & over by him & he’s still the selfish person!!
She wants kids, he DOES NOT want kids AT ALL! Doesn’t want the one he has. I hate to see her waste her life like I did with him!!
*My ex & I get along great, I know this will ruin it all by telling her. And I know us getting along is what is best for our son!
She deserves better, my son deserves better he’s even going on vacation away from him on Father’s Day 😢
Our son was also just diagnosed with ADHD, instead of helping me with treatment my ex is focused on his own life.
I know what people will think of me not sure I can bare that, especially my mother in law I am still very close with her. But I don’t think fair for him to get off either.
So do I tell her, come clean, tell her to run so she can start over with a better man! Or keep quiet for her & my sons happiness?
Thank you all & I regret what’s happening more than you know!! Please don’t be to cruel.
Sleeping with my ex-husband, do I break my silence?
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