If you take it because you want to, you focus on understanding it.
Most kids I know think of it as a process. You move this, you change that. It's not tied to any deeper understanding; even when the teacher tries, the students memorize the "understanding" and soon lose it.
When I've gone back to re-learn something I forgot, or learn something that I never learned, I may immediately focus on the technique that gets me through the bit of text I'm struggling with. On the other hand, more often than not I realize what the math is actually doing. Never got, for example, line integrals when I was around 20. I relearned the process, and then stopped to ponder what, exactly, the math meant. An "ah-hah" moment later, the math made sense. I'd learned the steps, but now they were exactly the steps that made sense in exactly the right order.
Same for Euler's equation. And a lot of other mathematical "stuff" I learned well 35 years ago and forgot slowly until around 5 years ago.
Really have to go back and look at diff. eqs. They'd probably actually make a great deal of sense now.
If you're adept at self-education, I'd track down an open-access course that can just be downloaded. A number of schools have them available, there must be some meta-site that would point you in the right direction. If it's algebra and pre-cal, that might be best done through a local CC.
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