its impossible to prove anything on a deep enough level. You cant even prove blue is blue in reality. Certainty is just a weighing up of probabilities. if you think its 51% likely that you are an athiest, call yourself an athiest. if its the other way around, so be it. Just the odds of it being exactly 50/50 in your mind are highly unlikely. i'd say just lean towards whatever you feel is most rational.
Agnosticism is not a valid alternative to theism or atheism. Gnosticism concerns KNOWLEDGE, theism concerns BELIEF, they are two different things.
You can be an agnostic atheist or an agnostic theist. You can't simply be agnostic. Sure you don't know if god exists, but do you believe in god? If you don't currently believe in god you're an atheist. If you don't know what you believe you're ______.
You can't "disprove" the existence of Xenu or Santa or Russell's Teapot, but you don't go around acting as though they're all perfectly reasonable ideas either. You acknowledge the negligible chance that they exist, and then operate on the assumption that they don't.
Going around pretending that Xenu is a possible alternative is dumb as fuck, and Yahweh (or any other myth) is no fucking better.
It's a binary position. You can't not believe in god and not not believe in god. EVERYONE is either theist or atheist. Agnosticism is a non-issue, nearly everyone (atheist and theist) is agnostic (doesn't know for sure).