Is cheating an option? Other than that, I don't have much advice other than practice. If you're struggling with a concept, do problems relating to that concept until you can get problems correct without looking up how to do any aspects of it. Though, this may not be too practical given your short amount of time.
You can try memory tricks like using mnemonic devices, acronyms that can be pronounces as words, memory palaces, etc. You could chant formulas or mathematical concepts to yourself, or maybe try to make a poem or limerick out of acronyms that rhymes so it can get stuck in your head easier.
If you're pressed for time you can try to learn what's you suspect will be most important and hope. You can try watching online videos, like Khan Academy (these are good for memory but bad for time, unless you skip around or only use them when necessary).
When preparing for tests that are coming up right away, you got to think quality vs quantity. Sure you can run through a list of everything you need to know quickly, but will you remember all that? You might need to sacrifice some concepts to dig deeper into more important ones.
Also, before a test, it may be better to get more sleep then to spend more hours studying, because memories get cemented into long-term when we sleep. It has also been speculated that listening to information you've already learned somewhat while you sleep makes it more likely to stick. I'm not sure if that's true, but it didn't hurt. I listened to Crash Course History videos in my sleep the night before my AP U.S. History exam (obviously I studied before that, but still) and got a 5.
Speaking of that, I studied for my AP U.S. History exam in one day after being a terrible student and not paying attention the entire course, and what I did was I watched Crash Course History and some other guy's videos sped up (to save time) and took as many hand-written notes as possible while watching them. Hand-writing notes makes you more likely to remember the information. Don't type. Write.
That's a little different than math though. History was more pure memorization, but with math you have to adapt more. Still, while doing problems, do them on paper.
Sorry this comment was all over the place. I was coming up with ideas as I wrote. Also, if you can't excel in math, that's ok. Holding yourself to too high a standard can be bad for you. Good luck :)