So I’ve been doing prep for the SAT and, as of today, I feel like I’m in a massive hole. My first score was 1440, not bad, I thought I’d improve, but since then, 4 weeks later, it’s consistently gotten worse, from 1440 to 1390 to 1360. I went from 710 in math all the way down to 660. I don’t know what to do, I don’t know if I’m just stupid or what. I go over my errors after the tests, particularly for math, and I get all the explanations and processes, even redoing the problems that way to make sure I understand them, but I’m still just falling. My ACT was 33 (didn’t prep beyond taking 2 tests, 35 on the English things, 28 on math), but I can’t seem to get even near to a 1450 on the SAT.
You have a 33 on the ACT. That’s a great score that is good enough for top colleges. So you know you aren’t stupid. It’s very disingenuous to even say that. The SAT clearly isn’t the test for you. Stop. You are done.
Hi TheTrueKing! You are not stupid; I think you need to take more practice tests. Even though I am not taking the SAT anytime soon, I have taken the SSAT twice and barely improved by 20 points. This was because I never studied as much as I could have. Your ACT score is great and your SAT score is decent as well. You seem to be getting your mistakes and understanding your problems, which is a good sign. To get your score higher, all I reccomend for you is practice, practice, practice and you will have an amazing score! Hopefully this helps and good luck! 
As @MajesticMeg said, definitely do not just throw in the towel because you happened to do… “badly”. If understanding the concepts isn’t the problem maybe it’s something else, like not reading the questions carefully enough.
Update: followed @MajesticMeg 's advice, bumped it to 1510, I’m delighted!
Final clue for the future. Don’t just check your answer for accuracy and do look over incorrect responses and learn why they were wrong. You newer know which way the question is coming
Wow that’s fantastic! Good job!