Short story: How do I learn math and retain it?

Long story: I'm going for a physics degree at university and I love the subject. Like, I've been reading the Feynman lectures and I get all jittery and fantasize about the possibilities of everything I learn and every thing I do learn loads me up with 100 other things I need to explore. Just started the semester with Physics 2 starting at two-slit interference and the concept excites me like everything else.

However, the thing I keep seeing as I read more like in my uni physics textbook, or A Treatise on Electricity and Magnetism by James Clerk Maxwell, is how horrendously terrible I am at math. Like I feel I'm beyond garbage mathematically for where I am conceptually in my studies.

I took your standard, Algebra I & II, Geometry, Pre-Calculus, Calculus I, II, and III

and I've read, on my own time, Calculus Made Easy by Silvanus Thompson twice now. I also brush up on integrals, trig and such in classes such as Physics 1 or Circuits or Statics.

However, I have to take a Differential Equations class at some point and so I bought myself a well reviewed book on it to practice before I take the class and I'm utterly disgusted by how much I have forgotten of things I've reviewed a million times like integrals and of how Diff eq just uses the information like it should be second nature.

"Here's a elementary problem so I wont even solve it" (inb4 triple integral using partial fractions)

And my uni Physics book isn't any better, I'll completely grasp the conceptual information but then they start proving an equation and I get utterly lost in simple algebraic maneuvers.

I had to look up to find out that x/y/z is the same as xz/y and stupid shit like 5 * x/7 ... wait ... do I find a common demononator? 35/7 * x/7 wait that isn't right and just made it more complicated anyway... oh fuck 5x/7 god damn it.

or I'll spend an extra 5 minutes thinking about the derivative of e^x or whatever before I realize I'm an idiot.

Basically I know a lot of math, but it's never been polished. I randomly learned a bunch of shit because my teachers told me to learn it and I never decided to actually allocate space to making everything a cohesive unit until like half way through calc 3. Now I'm like fuck, what do I do.

I'm at a point where I'm far enough behind that every time I do math I have to open a fucking reference manual and I'm far enough ahead that If I go trudging through algebra books I'm going to blow my brains out.

How do you guys learn math? What's the hot strat on the market. Do I write down all the shit I find I have no idea how to do and then make flash cards? Do I just yolo it and just keep looking shit up on Google or wolfram alpha whenever I forget a high school level maneuver? How do I fix myself.

Also, fuck online homework programs. How the fuck am I supposed to know what you fucking want as a final answer.