I see Flashbacks and Backstory as two different things. Your character’s backstory is her past, the details that inform who she is, her values and likes and dislikes, etc. Flashbacks are memories that sneak up on your character, unbidden, often unwelcome – little nuggets of her past that, for whatever reason, flash through her, a surprise.
Backstory does create it’s own challenges: how much to include, and where to put it in your story? Like a flashback, inserting a lot of backstory can slow down your narrative, as it isn’t moving the current plot points forward. However, it can inform the plot, the character, make everything more charged than it was before. You may have a lot of details about your main character that you find really relevent and fascinating, but what you have to ask yourself is, what does the reader really need to know?
When I started writing about Catherine Howard (please bear with me if I’ve already told you this) I wrote about her entire life, starting with the repurcussions of her mother’s death when she was ten years old. I followed her to the home of the dowager Duchess of Norfolk, her first romantic dalliance with her music tutor, and her later secret engagement to Francis Dereham and his midnight visits to her bedchamber. This took about 190 pages, and she hadn’t even arrived at court and fallen in love with Thomas Culpeper, or caught the King’s eye. While her backstory was interesting to me and certainly necessary to know, my readers didn’t need to read it all. It was enough for them to know that she had these romances in her past, which she would have to hide when the king chose her as his bride.
Don’t underestimate a good, juicy secret; maybe something your character is trying to hide. When it’s time to share, include only the pertinent, vivid details with your reader – not, say 190 pages worth.