You're all figments of my imagination

Is it possible to prove one's existence to another person, or to prove that one's reality is the true reality?

To break from everyone else in this thread, I say no, it's not possible to prove one's existence to another person or to prove that one's reality is the true reality. There is always the possibility of hallucination, an evil genius, that our real bodies are used by computers to harvest energy, etc.

Descartes's answer relied on the existence of a benevolent God, but unless you can prove the existence of such, that does not work.

If not, is there anything that can't be chalked up to simply being my imagination trying to trick me into believing something I imagined as real?

If anything, the existence of yourself ("I think, therefore I am") and perhaps logic and by extension, math.