What are good resources for learning Linear Algebra from scratch? I'm taking my second LA course, and I'm a math major but I've really only done Cal 1, Cal 2 and LA 1 before this, all of which were mostly just computational. But now, with LA 2, I don't understand how to make sense of what's going on, and I get it intuitively, but I'm really not sure how to prove anything or why it should be proven the way my instructor does it. 

Anyway, I really need something that could help build the more abstract concepts starting from like vector spaces and direct sums and stuff, because I'm just really, really close to failing this course. Please helppp!