You may have been able to remember a lot of math for a short time, but you ma have lacked understanding.
When you talk to people who excelled in school mathematics, they might be perplexed by your plight, or they may think you just didn't stick with it long enough. They might mistakenly tell you that you're just not good at math. However, traditional approaches that many of us experienced in school often especially privileged people with good memories or an ability to see patterns quickly, which might leave many of them to wonder why everyone does not come out of math education with the same insights they did.
The phenomenon your talking about is addressed in the Learning Principle of the NCTM Principles and Standards for School Mathematics. That is to say, the phenomenon you are describing is a familiar thing to mathematics education researchers, and is something often seen when an approach to mathematics focuses on rote learning and memorization. Stylianides and Stylianides (2007) wrote:
An important point set forth by the LP [Learning Principle] is that
memorization of facts or procedures without understanding often
results in fragile learning. This remark corresponds to research which
has shown that mastery of facts and rote performance of procedures
are not sufficient in thinking mathematically (Schoenfeld, 1988),
getting the right answers does not necessarily imply mathematical
proficiency (Erlwanger, 1973), and learning computational formulas is
a poor substitute for developing understanding of the underlying
concepts (Pollatsek et al., 1981).
The learning principle, stated by the NCTM, can be found here. In short:
Research has solidly established the importance of conceptual
understanding in becoming proficient in a subject. When students
understand mathematics, they are able to use their knowledge flexibly.
They combine factual knowledge, procedural facility, and conceptual
understanding in powerful ways.
There are a few models for understanding this phenomenon. One is that your view of math becomes more simple over time as you understand concepts that draw different aspects of mathematics together. It's a process of abstraction in which you have to remember fewer things, because abstraction compresses knowledge (that's one way to look at it). Another way to look at it is that greater understanding is not just about knowing more things, but it's about the connections among those things that you know. Without these connections, the things you have learned would remain in isolation, unuseful, neglected, and eventually (possibly) forgotten. A representation of this idea is in diSessa's (1988) Knowledge in Pieces, in which he talks about how people can have intuitive physics knowledge, but not a way of bringing that knowledge together into what we would see as understanding. Imagining these things in a sort of network, expert knowledge might look like a very efficient set of connections among ideas, tailored to the types of situations most useful to someone in a particular field (whether it be math or science).
OK, but how do you become more expert? The Learning Principle is meant to address that by promoting the notion that teaching approaches should be mindful of how student understanding is not a given (or a goal, even) in some approaches to teaching mathematics. It tells us that avoiding the fragility you describe requires a conscious effort to make understanding an educational outcome, not just a hopeful result of the application of memory or a practiced facility with procedures. So, seek to learn from people who take this view and the results will be a less fragile knowledge of mathematics.
One notable example, is Jo Boaler, who just happens to have an online course in which she addresses some of the issue of learning mathematics.
How to Learn Math is her attempt to educate teachers and parents about the ongoing process of learning mathematics. It's a good start if you want to understand what you or your daughter need to do to learn for understanding, and what teachers can and should be doing (based on the last couple of decades of research). You should also get some idea of next steps to address areas of mathematics you're interested in.
Hopefully, this has given you a way of understanding that your experience is not all that unusual, and can be addressed. But how you learn is important in how you will be able to retain and use your knowledge.
diSessa, A. (1988). Knowledge in pieces. In G. Forman & P. B. Purfall (Eds.), Constructivism in the computer age (pp. 49–70). Hillsdale, NJ: Erlbaum.
National Council of Teachers of Mathematics. (2000). Principles and standards for school mathematics. Reston, VA.
Stylianides, A. J., & Stylianides, G. J. (2007). Learning Mathematics with Understanding: A Critical Consideration of the Learning Principle in the Principles and Standards for School Mathematics. Montana Mathematics Enthusiast, 4(1).