You understand it well, but draw the wrong conclusions. 
Have you ever been abroad? In another country not understanding that culture? That was his experience. His Meditationes is very funny to read. One of the last philosophical writers who wanted to entertain their readers.
He believed in practice in the external world of course, but could not prove it philosophically. One of his other conclusions was that people should not be overly certain about their own beliefs and better listen to what someone else had to say (and if I remember correctly) we all had to understand better that someone with a different belief would really belief that. And that we can not prove that people with other beliefs go all wrong.
Now does it become difficult for me, because I do not doubt it at all. I will try however.
Noone can claim to have an objective perspective on reality. Although practically it works all quite well, if we are very precise, then must we conclude that every perception is preceded by a will and a capacity. Descartes was living in the Netherlands (Amsterdam to be precise), when he wrote his Meditationes and part of his inspiration became from his amazement how Dutch people differed in cultural beliefs from what he was used to and that those Dutch people defended their 'strange' beliefs with the equal certainty he had about his own beliefs.
He was very impressed noone doubted about the certainty of their own beliefs. That was his starting point. That implied that he had to leave behind all certainties, hence also all things he knows how to perceive. He must leave that certainty behind as well. Once left behind using the method of doubt, can you never get it back again.
Yet, he remembered his starting point: the certainty of different, mutually exclusive beliefs. Which were true? Could he get at some point, now that he had found the ultimate starting point of reasoning: the doubt of the doubt is unmistakingly the same as the doubt itself. He knew something with certainty, beyond any doubt so to speak: the first thing being the doubt itself, the next thing the subject that is doubting, and the realisation that that subject called itself me. Slow by slow did he try to rebuild all his knowledge of the world based on doubt and try to prove what was certain and what can not proven to be certain.
And of course, it has not to be proven to you, only to me. :-)
Okay, maybe I don't understand you, but it sounds like you're saying that having your beliefs questioned could lead you to doubting the existence of an external world? That's so drastic that it could only be the result of insanity---or great drugs