How did I survive this shock?

You got lucky.

Electrocution isn't an exact science. There are multiple things which make a difference:

• The current which flows through a person depends on the resistance. That in turn depends on whether the skin is wet or dry, the area of skin in contact with the wire, and a lot of other biology things.
• The resistance of the things completing the circuit - whether you're touching a metal radiator, or standing on a rubber doormat for example.
• The path of the current through the body matters too, it is currents through the heart that are dangerous, so getting a shock leg-to-leg is less risky than left arm to right leg, for example.
• Release time. If you drop the wire quickly, there is less risk of damage. The automatic response to let go of the painful thing might happen quicker or slower depending on all sorts of biological things.
• Health. Some people are just more susceptible to shocks than others. This might depend on build, body fat percentage, or just a pre-existing heart condition.

And of course voltage, source resistance and frequency of the source you're touching also matter. All in all, US mains is in the range where it's not guaranteed safe, nor guaranteed to kill, so it comes down to the factors above, and a healthy dose of luck.

Don't try it again.

There is a big difference between a current that is guaranteed to be safe and one that is guaranteed to kill.

In order to kill you there has to be either a current flow through the heart (eg. from hand to hand but there may be other paths, such as hand to leg) and/or you have to cook your body enough to cause fatal tissue damage.

The most dangerous situation is if your hands are wet (or sweaty) and the voltage is high, and there is a path through the core of your body. Avoid working on live voltage, and if you must, keep one hand in your pocket. Or, as an Engineer classmate of mine who worked for an electric utility advised, keep both hands in your pockets and let the technicians do it.

Milliamps can kill you, but that doesn't mean it will kill you.

There are countless variables involved, one of the most important of which is your skin resistance (which limits current significantly if your skin is dry). Also, it sounds like the current flowed through your arm, down your side, and down your leg to ground (that's the easiest path in most cases to ground).

If you had touched the neutral with one hand and the live with the other, the easiest path for the current would be across your chest, which means across your heart. This is where things get dangerous. If current flows through your heart it has a higher chance of causing fibrillation (your heart stops).

That's the idea behind the one-hand-in-pocket rule when working on live electronics. If you keep one hand in your pocket and only poke around with one hand, it helps prevent you from coming into contact with the live circuit with both hands which provide a path across your heart.