I have this guilt, too. (Although, in my case sd is the issue, she's dangerous! 😬) My kids are angels. I moved heaven and earth to get them into this world and would do it again, but if I had a magic wand or a time machine, I'd make some changes... I didn't fail, per se, but I also have to accept it wasn't my best work 🤣
I have the same exact feeling often. From the beginning, I told my boyfriend that if we ever break up, it will be because of her. I would have my daughter all over again. I just wish I understood what I was getting into. But we have our good weeks and bad. Hopefully, you're able to communicate with your partner about how you feel and set some boundaries up. It's so hard, you're not alone.
I'm sorry you're feeling this way. I feel this often too. Just here to say you're not alone. 💗 Blended family is like being on a never ending rollercoaster! 🎢