Is there a difference between when a person says 'I exists.', compared to the same person saying ' I'm existing.'? Is the person saying at the one time 'I exist.' saying he exists at a certain moment and has the 'functional capability' to exist for further moments in the foreseeable future? And when the person says I'm existing, maybe this is like the former statement yet emphasizing that the 'existing' is a functional capabilty to exist that is ever continuing , like a self-sustaining quality.
You cannot be certain that you are existing as your memory may be deceiving you, only that you exist at the moment when you are asking the question do 'I exist.'
And you cannot be sure about that ether. You are basing that you exist on logic (I think therefore I am) that you have learned through observation (by using your senses) so it's unreliable ether. We are completely in the dark, sorry.