If I were you, I would invest time understanding the foundations:
- Infimum & supremum
- Continuity & limits
- surjective and injective function
- Bezout's identity, congruent numbers
- Linear maps, linear dependence/indepence, dimension, basis
- Basic set theory notation
Definitions, proofs and propositions: this is the holy trinity of math. (Also known as blood, sweat and tears.)