Ahh, the countless hours I've spent in bed at night, writhing in disgust and unable to comprehend why my SO would reproduce with such a miserable person twice. "Why didn't he he have any standards??!" I have frequently asked.
My BM is not an impressive person. My SO's family is very large, and none of them had a single kind word to say about her. I have to search long and hard to find any redeeming qualities. She is selfish, closed off, unmotivated, has a history of being emotionally abusive and cheating. Drab personality. But my SO didn't want to have children with her. Both pregnancies were accidental (of course we're happy with and love the SKs now).
My partner has told me the story many times. He tried to break up, but found out she was pregnant. So they tried to have a happy family. I remind myself how those years he was very unhappy, how they don't have any hobbies or interests in common, how they seemed to be very incompatible, how they lacked intimacy and friendship and respect, how she mistreated him and put him down. He is always willing to reassure me if I am feeling insecure about it, and he says there's nothing for me to be jealous of because they didn't really have anything good going on. I knew him during this time, sometimes when he would open up about it I could see the desperation and the pain. I don't forget things like that.
I focus on what we do have, because our relationship is amazing. I remind myself that the closeness, friendship, intimacy and love he feels with me, he has never felt for anyone else before. We are pregnant with our first child, and he actually wants to do this with me, which is something he has never felt for anyone else. I can feel the conviction, commitment and passion when he looks at me, hear it in his voice when he calls me his best friend, and see it in his actions when he works his ass off for our family.
BM really screwed herself over by tearing down and mistreating such a wonderful man who is willing to give his entire heart and all of his effort if he is only duly respected and loved in return. I don't understand how someone can take an angel and throw them in the trash, but that's what she did. Basically, I can be confident that we have a very healthy, happy relationship and I focus on that. (Also, no my SO is not perfect and I acknowledge that he contributed to their marital issues too. But he isn't abusive, and he isn't a cheater, and I know he worked hard trying to make her happy. To no avail.)
This question hits close to home, as I've struggled with this a lot over the past few years, but I can happily tell you that it does get better. But I know that pain all too well. Focus on what you two have together. Your relationship is unique and can not be replicated.