Unless you're an astronaut, you could not have observed a very large segment.
On the contrary, if I were a world traveler I could claim to have seen damn near all of it. I'm not making that claim though.
You can only observe a few miles at a time. You cannot observe a significant portion from the surface to examine if there is any gradual changes to the surface such as a curvature. You would examine many segments independently, all of which are an insignificant portion, and too many to consider what shape they all form when combined.
Unfortunately, any segment you have observed was not large enough to logically draw any conclusions given the size of the earth.
You keep saying that but you haven't yet backed it up.
Well, do you disagree that the earth's surface is 24,900 miles long, or do you disagree that there could be subtle changes such as a curvature which could exist, but not be observable from our field of view on the surface? Your philosophy seems to be you only believe in what you observe yourself, so I can't support my claim that the earth's surface is 24,900 miles long, but you would have to go survey the earth yourself, and the oceans are quite large. If you want evidence that curvature of a round earth would not be observable from the surface given the circumference, I can provide you with geometric equations.
On what should I draw my conclusions about the shape of the Earth, if not what I observe of it?
With your philosophy, you can't draw conslusions about the shape of the earth, unless you do some experiments to see if your sight is obstructed by the horizon and allow the assumption that light travels straight, or purchase a flight on a space shuttle or high altitude jet.
You have in incredibly closed mind.
That makes no sense. If I had an "incredibly closed mind" I would never have come to the conclusion that the Earth is flat in the first place.
Regardless of what you do believe, you are closed-minded because you accept only what you can observe first-hand. Just because your belief is uncommon does not mean you are open-minded.
I believe that statement you have made many times is entirely untrue. You will never accept any evidence which contradicts your belief. Any such evidence you would claim is invalid/untrustworthy.
If it's secondhand, why would it be the logical thing to regard it as otherwise? Do you believe everything you're told?
No, but to claim you would believe in a round earth if provided evidence, then claim anything second-hand is untrustworthy is a contradiction. Nobody could provide you with first-hand evidence that the earth is round, so stop making this claim if you refuse all second-hand evidence.
Second-hand evidence is acceptable when it is very unlikely to be fake. Of course that is very subjective, but some sources must be considered trustworthy if we are to build on their knowledge/observations and further our understanding of the world/universe we live in.