Question: Sorry if this question is dumb, but I don't seem to have a grasp on it.

Suppose you are on a rock in space, with no external forces acting. The rock attracts you with a force given by

$$F=G\frac{m\_1m\_2}{r^2},$$

and you also attract the rock with an equal and opposite force. The ground exerts the normal force which keeps you stationary, but what stops the rock from accelerating, however small that be?

Answer: 
Just like the ground exerts force on you stopping you from accelerating, your legs push the rock stopping it from accelerating.
Gravity pulls together. Electrons on the surface of your & the rocks molecules push apart. Electromagnetic forces are gigantically bigger, but usually cancelled out by charge carriers moving to ensure things stay electrically neutral.

On a small body, gravity will be negligible, so even very little movements of your feet will see you bouncing away from each other. This is why the [OSIRIS-REx](https://en.wikipedia.org/wiki/OSIRIS-REx) and [Hayabusa](https://en.wikipedia.org/wiki/Hayabusa) asteroid sample-return missions didn't 'land' in the conventional sense.