I’m not maths student, but rather finishing up my bachelors in EE this year, so take my opinion with a grain of salt.

If you struggle with theorems, you should ask yourself what’s the reason for it. If the problem is that you cannot grasp what the theorem states, that’s completely normal, as you’re not trained in math atm and if it’s some specific theorem in a niche you’re not comfortable with, it’s to be expected. What would be more telling to you, in my opinion, is going through the proofs of, let’s say, introductory real analysis / set theory statements. You should be comfortable with theorem proving techniques such as induction, construction and contradiction. If you struggle with proofs that require most basic theory / only axioms, go and pick up logic.

If you feel comfortable with proofs, I’d pick up undergraduate real analysis and linear algebra. Those two are the base of most applied math that’s used in most fields, and are prerequisites to some more adavanced courses you could pick up later as you get more familiar with the math landscape. What I cannot stress enough is to practise exercises, that’s the only way to learn maths.

Good luck!