I also donno why I'm getting 519 when it's horizontal, maybe cus the analog reading is 0-1023 and 519 is near the middle
Yes, 519 is very near the middle of the range, but the 0..1023 range represents a range of 0 to 5V, so mid-range is 2.5V, which is not what the 0g point of your accelerometer should be at if you're powering it from 3.3V (the maximum supply voltage for the device you're using).
It should be more like 338, as noted earlier.
However, if you were powering the accelerometer from 5V (and thereby potentially damaging it), then perhaps the 0g point would be at 2.5V
Have you checked your wiring?