There are fast math techniques that teach separating numbers into ones, tens, hundreds, etc. and then adding it all up at the end as a way to do quicker addition and subtraction. I can do up to the hundreds like that sometimes but beyond that I can't remember the ones or tens by the time I finish the thousands.
I should probably also mention that I have ADD and don't take any meds for it, so keeping things in short term memory is a challenge. I'm also not a visual thinker so I have a constant running monologue going in my head and it jumps around a lot. Thoughts are (usually) connected, just sometimes the connections between two thoughts go by quicker than I can conciously process without sorting back through and actively searching my head for the connections. And that's just the internal distractions. Movement, light, sounds and smells can all distract me as well. So, yeah, remembering things short term is sometimes a struggle. And of course things have to stay in short term memory in order to get into long term memory. I could remember the quadratic formula after precalc because I'd learned the process of creating the formula. Knowing the method gave a reason for the different parts of the formula and therefore a connection to each other, which made it easier to remember. If I forgot a part, I just had to go through the process of creating the formula in my head and I could remember it again. I did the same thing with other areas of math and with various areas of programming.