I am sorry you are going through this and I feel you WILL get through and begin to feel like the winner you really are.
Many reasons for you to feel cruddy. Being married for 20 years is a long time and a LOT of investment. You've been a good wife and what are your "returns"? You are not getting out of it what you invested in it and that sucks and hurts at the same time. It's like losing hard earned money invested on a retirement account: really, really sucks, big time. But, I am not equating what you are going through and have gone through with money. Your feelings and self esteem are way more valuable than that.
When someone chooses addiction over us that's one thing. But when they ALSO choose another woman over us, that's a triple blow. It hurts so much it's hard to describe. We understand how that feels and how much it hurts, and on an intellectual level we KNOW it is not a reflection of our WORTH, but it is STILL hard to swallow. It's about feeling rejected, yes. But it's also about betrayal, loss and getting hit with more it all on more than one level.
I was once with an addict who was addicted to attention. And, reality is I could never give him enough attention. I was working my butt off in a full time plus overtime demanding stressful job and I was maxed out at times. I needed love and support myself; the right kind of attention. When I could not give him enough attention he sought it elsewhere. I was the bread winner and paying the bills. The relationship did not work out, it was sort of doomed from the beginning. It might have been different if he was also working and bringing in money. For one thing, he would have been busier like me with less time on his hands. He might have not felt such a need for attention, I don't know. But anyhow, I learned a lot of valuable lessons from that one relationship alone. He wanted it both ways or all sorts of ways, I dont' know. I wanted the good money I made and respected that about me, but he also took advantage of it and then took it for granted that I would always have a job, always work and always bring in good money. What a deal, huh?! But he also wanted the thrill and excitement of new attraction and then new love with someone who had more time on their hands to give it. I just felt so cheated and used and betrayed on so many levels it's not even funny. I worked so hard! And he liked the money but didn't appreciate how hard I worked for it.
That someone cannot seem to respect you enough to stay clean and sober and clean and sober for your mutual children really sucks, it really does. But you are a very respectable person who deserves love and respect. Stay strong.