Big score variations in practice tests??

I'm taking the October 4th SAT and am feeling a little panicked. I got a 1420 on my very first practice test w/ no prep but have not done nearly enough practice. In a july practice, I got a 740 reading and now on Bluebook 7 I got a 680. I've gotten a 790 on math practice earlier this summer and got a 680 just now. It literally just depends on the day and usually I can't tell that I'm getting a bunch of stuff wrong. I've gotten a 1490 on a practice a few months ago, and my most recent test I got a 1360...

I'm trying to get a 1500+, which I was told was possible, but am starting to feel straight up stupid. Any tips for studying for future tests? Dw I know I'm already cooked for this weekend