This relationship bugged me for awhile but not for the same reasons...mick being married was part of it....the fact that it was under lb's nose and all that, but as I learned more about FM it seems that they all kinda had relationships under each others noses. It was just part of living in the bubble that is\was FM. CM did that to john...SN was with some of Lb's associates and Mick....Lb seems to always have his girlfriends around under SN's nose. So that seems to be part of the culture they had going on. Sure, they were both messed up on cocaine, but I dont think that points to reason for the break up.
What gets me, and seems to not effect anyone else, ( as i have brought it up many times ) is that Mick went after Stevie's best friend and she ended up marrying the guy! Then he spills the beans about the whole thing in his book.
First of all, Sara broke a cardinal girlfriend rule...NEVER date or sleep with ur friends men past or present. ( at least thats a rule in my world)
Then Mick's kiss and tell.
As far as the reasons she broke up with him...who knows

I think since Mick went and told she had to come up with something to say and the "career thing" or "for my band", has alway been her rote answer regarding marriage and children.
In this case though I wonder if the reference to the band breaking up is "code" for LB would get pissed and leave the band
Personally I think Stevie has commitment issues and she used her career as an excuse, and thats cool with me. I mean there is no law that a women HAS to marry and have kids. Not that SN and Mick would ever have progressed to getting married..( okay yikes..shakes image from my mind

)
Though I think it is a regret for her not having children, I don't know that it is her her biggest regret. We all have regrets she is no diffrent. She made a choice and she gotta live with that, just like the rest of us.
It doesnt mean she moaning for children now, I think she is fine with her choice. Still...she would have been awesome as a mom its kinda sad, to me.