Mathematical proofs didn't click for me, so much so that I ended up failing my Discrete Math class because of it. Before I retook the DM course again, I took a Proof Workshop but I doubt I would've done good without the following books:

* Richard Hammack's The Book of Proof [1]

* Mathematical Proofs by Chartrand, Polimeni, and Zhang

[1]: https://www.people.vcu.edu/~rhammack/BookOfProof/