It really depends on what you did. Some mistakes you can’t buy your way out of. Like cheating, or stealing something, or saying something, like, “my ex was better in bed”.
Sometimes you just have to take your lumps and move on and wait for time to heal the wounds.
If its something like you forgot your two month anniversary, than let her cool off. Its called an anniversary because its suppose to be annual. Not monthly.
If you simply forgot to celebrate her birthday, than find a personal gift to give her to make up for not getting one in the first place. If she likes scarfs buy her a soft beautiful scarf of a color she doesn’t own or in her favorite color so she knows you are paying attention to the things she likes.
If she does love flowers, buy her her favorite. Don’t go for the generic flowers.