I took the SAT last year as a sophomore and scored a 1390 (I got a 780 on R&W but my math score was… yikes)
I’m pretty happy with my R/W subscore, & most of the schools I’m looking at will superscore. Do colleges take subscores from prior to your junior year?
Second, I really want to bring up my math score. I’ve always done very well in my math classes, but I can’t seem to master SAT style questions. I’ve bought a prep book but it’s so huge that I just don’t know where to start. I’ve heard good things about Khan Academy & I’ve used it & loved it in the past for some of my AP classes. How would you recommend studying for the SAT if I’m mostly looking to fix my math? I have trouble motivating myself to stay focused for long sessions.
I can’t comment on your first question, but I have some advice for your second question. The math questions on the SAT are designed to be tricky to understand, but as long as you understand the fundamental concepts and find your own problem solving strategy, you should be good to go! First, maybe you could take a look at your SAT score report to see which math concepts you missed. This way, you can focus on what you need to improve on the most. Second, maybe you can find a way to review this content, whether that be through your math course textbook, Khan Academy, or any other resource you know about. I think the most important thing to do to prepare for the math section is practice, practice, practice! This way, you can become familiar with the same types of problems and find the best way to solve them efficiently. Maybe you could use PSAT and SAT sources from the College Board (including the official sample questions, student guide questions, and Daily SAT app archive). To motivate yourself to study, maybe you could study in a small amount each day. For instance, you could dedicate 30 minutes each day to learn a new concept and do some practice problems. Hope that helps! 
Uworld helps for math, but it costs a substantial amount