Do you feel like they exist?
Boundaries? No.
There's an obvious boundary of sorts between what I know and what I know not but it's really a formal distinction, i.e. an abstract boundary, like between being and non-being. So I wouldn't say I feel like it exists.
Other would-be boundaries are integral parts of one perception field or another, between hot and cold, between hard and soft, between red and blue etc. So, there you could say I feel the boundary as a perception. I couldn't help it, obviously. Yet, I wouldn't say I feel like these boundaries exist. Rather, there are boundaries within my perception and they are real enough, and I interpret them as significant in terms of the reality perceived, but not going so far as accepting that it is necessarily true that there is any real boundary beyond my perception. I'm agnostic in that respect.
Are you a dualist if you think they both exist?
Well, it seems that any conception of reality that would require at least two distinct qualities, not reducible somehow to a unique quality, would have to be regarded as dualistic.
Still, we can represent curves with inflection points where the curve is effectively continuous but the differential is discontinuous and marks therefore a boundary. Think of a continuous two-dimensional space, a surface but with an inflection line making a fold going through it: a cube for example, with a continuous surface all over but edges which are in effect inflection lines, marking the place where the differentials are not continuous, which allow us to say that there are six faces to a cube.
But one may argue that only the three-dimensional space within which the surface appears is real, the surface itself and the boundaries with said discontinuous differentials being only epiphenomenal.
I wouldn't know...
Are you a monist (continuous transitions allow for discrete boundaries, so discrete boundaries are part of continuous nature)?
I can certainly understand how perception could conceivably make a continuous transition appear like a boundary. And vice versa. So I'm not going to be too categorical about anything.
Is asking these questions going to make me any less screwed by reality?
You seem to be assuming that the whole of reality would have to appear skewed to us. I would say that this seems to me to be necessarily an impossibility. That is, at least a part of reality can only appear exactly as it is to us. So that in effect, we have to know that part of reality exactly as it is, even if, perhaps, we're not minded to pay attention or think precisely in those terms.
EB