If speed is not an entity in itself, but only dependent on other constant factors, how can the speed of anything (let alone light) be a constant? Am I completely missing something here?
What you are missing is that distance and time themselves are not constant. Both distance and time depend in part on the velocity of the observer.
1. You are on Earth, and determine that the distance to the Sun is 93 million miles.
2. You get on a rocket and start flying towards the Sun. You check again and find that the distance, not just between you and the Sun, but between Earth and the Sun is now only 90 million miles even though neither of them has moved towards the other.
3. You turn on your radio and call your friend back on Earth. He says the Earth-Sun distance is still 93 million miles.
4. You turn your rocket around and stop, now motionless between Earth and the Sun. 343
You confirm that the Earth-Sun distance is 93 million miles.
5. You turn the rocket back on to head home, and find that the Earth-Sun distance has somehow shrunk to 90 million miles again.
6. You arrive at home and check the clock. You've been gone 30 minutes.
7. You check your phone that you brought with you on the trip, which is still in airplane mode and hasn't synced yet, and it says you left only about 29 minutes ago.
The thing about the speed of light is that it's not actually light that's special. I've seen the speed of light referred to as the speed of causality, the speed of information, the universal speed limit, the cosmic speed limit, and probably a few more terms I haven't thought of right now.
The speed of light is the fastest that anything can move, and all things that have no mass will always travel at that speed. This speed is more fundamental than distance or time, and anything moving at that speed will always be measured as having that speed regardless of the measurer's movement.
This necessarily requires that people moving at different velocities will see the same interval as having different lengths, in both distance and time. This means that space and time contort in some strange and difficult to understand ways based on your movement, and Relativity is essentially an extended exercise in analyzing exactly what contortions are needed to produce an invariant speed. In any case, the effects of this for moving at any speed you've ever experienced are far too small to notice. 343