Michelle,
Have your tried talking to your teacher about your daughter's lack of confidence? Perhaps there is something going on in the classroom that is affecting your child, i.e. too much pressure. I would advise going to your child's teacher first, but also trying to get your daughter to recognize things that she is good at, even outside of school. Is she good at taking care of her toys? Does she like drawing or singing? By focusing on the things that she does like to do, you might be able to make her more confident overall.

That is a great idea, rkaiulani. I agree. Also, is it just that she works slow? I teach kinder and have a student that is a perfectionist. She works very slow because she is careful. Maybe you could have some fun and build some fluency with her. Have some racing games  to write letters. See if she can beat the clock. Give her a dry erase board or small chalkboard and quiz her on letters, numbers or even sight words. Also, you could ask the teacher for some extra practice material such as a small spelling list. You could have her write the spelling list on her own and time her. Tell her that the next day you want her to beat her own time and when she does tell her she will get a sticker on a chart you can make at home. When she beats her time she gets a sticker and when she gets 5 stickers or so, have another reward like extra play time, a small toy from a treasure chest or a new pencil. This works in my class! Hope this helps! Happy parenting!