Also, what was your reasoning for doubting that you exist? How was Descartes proof insufficient?

It's essentially circular. It assumes an "I" from the start. If you get rid of that assumption, you have to start with "something is thinking."

That's been acknowledged in philosophical circles for some time now, but I don't think many philosophers regard it as an important problem anymore. It's about as safe an assumption as you can possibly make.

If anyone wants to google this, keywords are "nonduality" and "Advaita". It probably deserves a memetic hazard warning, though.