The problem starts when the curriculum attaches needless importance to fractions. 'Mixed fractions', wth curriculum-authors? Similarly with adding and multiplying fractions. The emphasis is more on the procedure and getting the student to be able to implement certain procedures, rather than understand the underlying intuition. Let's take two examples: 1/2 + 1/4 and 3/4 + 1/7. For the former there is a heavy intuitive reason for kids to understand adding half to a fourth. If you think about the latter, you are just adding two mathematical entities with little physical relevance. Equivalent entities are 0.75 and 0.142857, there is no good reason why one should stick to fractions when you are dealing with hairy numbers like 1/7. Kids aren't told this, and it complicates issues. The answer that kids are forced to look for is -1-1-/-1-4- 25/28, which is as unintuitively clunky as 1/7. They are challenged to add increasingly hard fractions like 13/37 + 22/7, etc. At this point, kids who can grasp abstract concepts do well, and those that can't get discouraged.
I think physically intuitive math and abstract math should be explicitly identified and treated differently in curricula. Else, the result will be clumsy contrived attempts to find physical relevance to abstract concepts and weaker students falling behind. Dreary contrived problems must be replaced with fun puzzles. Puzzles share the attribute of abstractness-and-apparent-uselessness with portions of the math curriculum.
Somehow teachers and parents find it hard to accept that some topics in math curricula are purely in the abstract realm.
The other meta problem and perhaps a big cause of math anxiety is that the curriculum and system doesn't have much wiggle room to fail at understanding something. The most important quality for success in later life is perseverance when you're stuck or when you hit a wall. The education system teaches the exact opposite: "it is not ok to hit a wall or fail to understand something, you have to learn this right now, else you're done for!" There is very little flexibility to allow students to fail, persevere, and repeat. This is worsened by the needlessly frenetic pace of the academic year.
Edit: D'oh, got 3/4 + 1/7 wrong, it should be 25/28