I'm personally thinking that there os existence to "endless" things- the universe and time for example. You pointed the idea of the "circle" which is somthing we can comperhend and it can help us accept the concept of endless things, even if it's not really endless, yet - we get some feeling, if not logic, about the notion of endlessness.
Yeah - that sound irrational - but so quantum theory, and yet so far this theory is proved correct even though it's against human logic - how can an integer equal to 0 and 1 in the same time?! Yet it's possible and there are computers in early development stage implementing that logic.
So we can't fully understand it- we can just try to touch the idea with our mind- get some hold of it. just like in this lovely, somewhat abstract, photo - the mountains are in a red fog, just like our minds are fogged from reaching the comprehension of the meaning of endless, but we can see one mountain tops reaching out of the fog and partially and gently touching the far unreachable moon - just like sometimes, having the right metaphors and after drop thought, we sometimes manage to get a peep into the meaning of abstract and unreachable concepts.