Third grade is a huge leap academically and developmentally. There is an expectation of independence and time management that many children struggle to learn. This is also the first year of state assessments for most children and that increases the pace and breadth of curriculum maps.
I would talk with the teacher about having your daughter be the "helper" in the classroom. If this means that she's feeling pressured to get things done quickly so she has time to help, then it's not OK. If the teacher has a peer-tutoring program going on in the classroom, different story.
Third grade now is nothing like when we were in school. It's faster and more complex. Telling her that it's OK to make mistakes is a great start! I would also make sure that she sees you and your husband making mistakes and learning from them. Many times we keep our short-comings from our children and they need to see that we forget things, say the wrong things, do the wrong things and survive.
If she doesn't already have a non-academic acitivity, consider letting her join a club or something else outside of school that she would enjoy and that would help her find some balance.