If you want to get into "rigorous" mathematics, I'd probably go a path similar to the one I'll outline here, but YMMV.
You may want to pick a book on writing proofs to familiarize yourself with the concepts first, such as "How to prove it" (either the one by Velleman or Polya). Another good one for getting to some intuitions might be "How to solve it" by Polya.
Then, you might pick up any elementary book on Real Analysis, Linear Algebra, as well as Graph theory/some Algorithms. Most of these should be self-contained. These 3 areas should lay a very firm mathematical foundation, and other parts of mathematics will become a lot more accessible with them.
Just be mindful that you'll probably need a long time going through these books, and that's normal. If you gloss over things, you'll quickly miss important bits. It's not like other books where you can kinda grok things out of context if you just continue reading, at least for me. I wouldn't do more than max 2h per day, but be consistent if you want to see progress.
You may want to pick a book on writing proofs to familiarize yourself with the concepts first, such as "How to prove it" (either the one by Velleman or Polya). Another good one for getting to some intuitions might be "How to solve it" by Polya.
Then, you might pick up any elementary book on Real Analysis, Linear Algebra, as well as Graph theory/some Algorithms. Most of these should be self-contained. These 3 areas should lay a very firm mathematical foundation, and other parts of mathematics will become a lot more accessible with them.
Just be mindful that you'll probably need a long time going through these books, and that's normal. If you gloss over things, you'll quickly miss important bits. It's not like other books where you can kinda grok things out of context if you just continue reading, at least for me. I wouldn't do more than max 2h per day, but be consistent if you want to see progress.