I am almost certainly falling in love with my best friend
That statement does not sound certain.
If you truly love her, her past should be in the past and will not matter as she is with only you now.
You refer to her past as 'mistakes'. Does she consider her past as mistakes or are you projecting your values on how you expect she should feel?
Instead of being concerned with how you can deal with her past, if you truly love and cherish her, should cause you to question how you can measure-up to be the best partner she could ever want and deserves.