I am trying to backtrack and remember my math classes throughout my academic life and can only remember a time in 5th grade where I started subtracting from left to right. Somehow I also completely missed the fractions lesson in grade school and even today (sophomore in college) I struggle with fractions.
I am learning proofs now and the most difficult part for me is honestly the arithmetic. I can follow formulas and rules but when it comes to mathematical creativity, logic, and reasoning, I fall behind. It seems I am at least 2 minutes behind each topic in class when trying to understand the material compared to the rest of my classmates.
I want to develop a deep mathematical thinking and maturity and feel the best way to do is by filling in the gaps from the beginning (because I am not sure where all the gaps are).
I thought I could start with Khan Academy. Is this a good resource to accomplish this or should I look elsewhere?
For fundamental arithmetic, Khan Academy is the best, hands down. Although once you reach really complicated things, other resources online may prove to be better/faster, Khan Academy has absolutely everything you need. Even if you just do early math (Kindergarten - 5th, arithmetic, some fractions) it is extremely thorough. This link shows you the breakdown, and you can just go through the exercises (there are close to 500 types, each with 5+ problems).