When you have a very detailed character; good. When you run into this kind of problems because of it, you've made a character too detailed though. And when that becomes the case, don't try to force it after all. I too like making characters that are more detailed than they have to be, with details and habits only hinted at and a lot that's not directly plot-relevant. But the line lies somewhere in the grey area between keeping the character completely plot-relevant (Chekhov's gun) and writing several fluff scenes just to flesh out the character, this grey area is using the fluff scenes you have to add already and adding a lot where it wouldn't be fluff.
My advice, though I don't know what exactly you're planning to do, is to scrap a lot of those pre-story scenes. Not in the way that she becomes 2D, but in that you don't write out entire scenes just for one character. First, take all those scenes you want to make, and determine whether they can become simpler. Good chance that a whole scene describing some event can be summed up just as accurately in hindsight with a few sentences. A lot of these long stories might be summarised with just the end result, the journey not mattering that much. And others you consider flashing back to might not even be needed at all. (Don't be 'How I met your Mother'
) Let's take three events (I'll make up on the spot): 1. She's perfectionistic, 2. wants everyone else to be so too, and 3. can lash out when they're not.
1. you might want to just represent as her personality. This can also be done past the mid-story, people don't need to know how it came to be or how it affected her career's start. She can comment any of these things herself without flashback, like saying that she never would've gotten past the $xx0000 mark without reading her emails every 15 minutes or something. If it's something that's still her habit, find a way to put it in after the mid-point rather than needing to go before.
2. can be repeating itself. In this new town she got an apprentice or someone eager to be mentored by this once successful lady. Being perfectionistic, she will demand the kid to be just as perfectionistic and was a slave driver to this end. Flashbacks could be seamless to this end, or even invisible. She has two apprentices who are getting more and more frustrated (but who never seem to interact), and speak up. They have to do all these menial tasks with rediculous precision, but she doesn't trust them with anything important or interesting. Then one of them storms out, and it's reveiled that this one was actually a deja vu from her previous life. As sudden redemption in this new life, she uses this knowledge to prevent the other from leaving as well. This would add a flashback arc into the story without making it feel like such, and doesn't make the conclusion too obviously predictable.
3. Let's assume that the big mistake that she made was insulting the wrong person. Our perfectionist prepared a speech and all details for some prestigeous event where she was receiving an award or something. But some much more subtle antagonist woman kept pushing her buttons. Then, on stage and in front of everyone, when the stress and neurotic desire for perfection is highest, that antagonist pressed her buttons one last time and once too many. Thus, the Bad Mistake. Instead of having to completely flashback to it, make it a bit extra juicy with some details of violence but don't tell it coherently. Just have people comment about the main event (the Fake Wig happening, or maybe even The Nipslip. Well, you already know what her real Big Mistake was, what would be the most memorable part for the common man?) but never the entire story. And the MC isn't too eager to talk about it or fully relive the event. Just little nitbits at a time.
If you absolutely must have all the scenes you envisioned and this would make the story twice as long, it might make a good sequel in that case. Make your story mid-way, and once you finish and release it you can determine whether the people want to see her story before this one. If they don't, if you already told everything about her that they want to know, good. If they do, write it as it's own project.
(Edit P.S.: I also agree with what Wildershins wrote, their suggestion could also work well)