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No matter what you say 1+1=2. You cannot dispute this fact.
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That's because "2" is
defined as "that object which results from adding 1 to itself."
As other threads have brought up, math is axiomatic. Currently we are using the
Zermelo-Fraenkel axioms (or variants). They are taken as a priori true and cannot be proven or disproven. In fact, axioms are added or removed in different circumstances, and this has consequences on the implications of those remaining axioms.
The axioms used are chosen because 1) They seem intuitively reasonable; 2) they are provably consistent (see Godel); 3) they give results which seem to mirror the observed world. But they are still logical constructs, not isomorphic to Reality. Godel also proved this, when he showed that no internally consistent axiomatic system could be used to prove all statements true within that system. In fact, there are infinitely more unprovable statements than provable ones.
To recap, math is a human construct, very good and beautiful but not reality itself. As to "math existing after you die," maybe that's from another thread but I have no idea what you mean by it.