Daughter doesn’t understand why her dad and I spit and I’m running out of things to say

EDIT: the title is supposed to say SPLIT not SPIT.Hey Reddit. I’ll try to make this as simple and short as possible.My daughter is 8 and her dad and I separated 3 years ago. Recently, she has been incessantly asking why we can’t be together and why we broke up and why we weren’t happy. She’s been adding in things like she’s sad now (understandable) and that she was happier then (she was 4 when we split) and that she hasn’t been happy since.The last time her and I had a conversation I tried to explain to her that her dad and I weren’t happy with each other anymore and that we’re happier now. She said “well I’m not happier. So you care more about your happiness than mine?” That pretty much took my breath away. I didn’t think she could comprehend that concept and to be honest, I didn’t know what to say. I ended up explaining that we knew that ending our relationship was the best for everyone because eventually her dad and I would have gotten more upset and she would have seen us be unhappy. And by her recollection of events, it was clear we did our job right because she didn’t know we were unhappy.The problem is, I have the same answers and she wants to have this conversation at least once a week. It is very hard on me, obviously, because it leaves me feeling selfish and like a failure as a parent. I fear that continued conversations like this will lead me to lash out due to pure frustration. And I don’t want that.My daughter is very smart, she understands how to argue and with conversations like this, she has an answer for everything.I know I shouldn’t be upset but I don’t know how many more of the same conversation I can have. It’s like she’s completely unreceptive to my explanations. I’m trying my hardest to be diplomatic and explain in terms she can understand but I think we’re past that. It’s not like I can say “your dad left me and made me move out”. But part of me wishes I could because I have a feeling she blames me for all of this.Has anyone dealt with a similar situation? Anything I could say to her, a better explanation, literally anything. Thanks in advance. via /r/Parenting http://bit.ly/2Uiy6zP