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.)