Is Descartes's Epistemology a bit circular?

First of all, everything I will say next sounds really stupid when I repeat it to myself. Our class discusses what we know and what we don’t know, but does that really matter? When I stand in the middle of a street, and a bus barrels towards me, should I debate its existence? Should I move?
As stupid as Descartes’s philosophy sound, it does establish an intellectual framework I suspect Did Descartes change philosophy? Well, I don’t know enough to say if he did or if he didn’t. What I do believe is that Descartes approaches problems logically: he knows for sure only what he can see in his mind. According to Descartes, he exists because he is sentient and autonomous, and that he is unsure of the existence of anything outside his mind.
I do not question his sense of reality, for it aligns closely with my own. What I do question is his epistemology, especially in fields he feels are obvious to him. For instance, Descartes holds that mathematics is true whether it exists in reality or not. But if mathematics do not exist in actuality, how do its laws work inside his mind?
Descartes’s answer is that there’s something inherent in mathematics that makes it right. How does he know that? He imagines the laws to be so. But what mechanisms inside his mind make this true? Descartes cannot tell us because his philosophy allows him to reference nothing outside his mind, save for what he trusts. If he cannot reference anything outside of his mind, how can he truly know anything?
That’s my primary question with Descartes’s inward-looking philosophy. I have other questions, such as one concerning his proof of God’s existence. However, the above question stands out in my mind, and hopefully, we can have a class to discuss this in.