Existence cannot be proved (and therefore God cannot be proved). All your sensory perceptions could be lies. Buddhism considers "thought" or the perception of thought to be essentially another one of the "senses". Honestly you can't even prove that YOU are really thinking what you think you're thinking, (most computer AI though pretending to be sentient is not. your brain could be another example of this). Descartes should be corrected "I think therefore I BELIEVE I am".
To work backwards: Proving god exists requires belief that the universe exists. Proving the universe exists requires belief that our sensory perceptions aren't lies. Proving our sensory perceptions aren't lies requires belief that our mind isn't being tricked or manipulated. Proving that our mind isn't being manipulated is impossible (see the Matrix).
But to go one step farther, even the thoughts in your head could be placed there by some other entity. Therefore you can't even prove YOU are real on any level. At most you could say the fact that you think you feel things MIGHT prove that something exists in order to cause these feelings. However the extent of this being is immeasurable and undetectable given that you do not perceive it.