Warning: The following methods are purely theoretical and not recommended for actual use. Or is it?
Attempt to exist and not exist at the same time, like Schrödinger's famous cat. This method involves setting your consciousness to 'both dead and alive' while simultaneously being in multiple places, making it impossible for anyone to know where you are or if you even exist.
Travel back in time and kill yourself before you even decided to do it. This method involves creating a temporal paradox, effectively making the decision to commit suicide before you even had the chance to think about it.
Use quantum entanglement to erase your consciousness from the universe, leaving behind only a faint scent of despair and regret.