我该怎么教三年级女儿数学呢?

I got a big headache teaching my 8 year old daughter math! I felt painful!
She is a good reader. I think that is why she is in the enrich class of third grade. In her school, there is one enrich class in every grade. Each grade has 4 or five classes. At the end of the semester, the third graders will take a set of exams to decide who will qualify the gifted program--which selects only 8% of the students(one class every 3 schools in the district).
I think her math is the weakness. So I really wanted to do something to help. But I guess I was not a good teacher because my daughter often takes a long time to understand what I taught her. Both of us felt frustrated. My disappointment often showed on my face. She once cried, and said, "I know I am stupid."
Yes, I really hope she could get the opportunity to get into the gifted program. However, I do not want to let our lives be ruined by it.
I began to think whether I was teaching her in the wrong way, or I was expecting too much from her about how fast she must master a concept or a skill. I really don't know what is the average time of third graders understanding a new concept such as long division, or simplifying a fraction, or changing a fraction into a decimal. For my daughter, I gave her two lessons about long division, each 20 minutes long. Only at the end, she understood it. She then spent two weeks to practice long division, 1 or 2 problems every day. she seemed to master it. But one months later without any practice, she forgot how to do it!  For simplifying a fraction, I drew diagrams, and then taught her step by step. It took her 30 minutes to understand it. Is she too slow?
If she is really slow in math, I am thinking I need to stop pushing her. 
I got tired. But I am still hoping if it is my wrong way of teaching that caused our frustration, then she still has a chance.
 
Help! Somebody! Please!