Riddles in the Dark

I am the enemy of progression. I am an idea serial killer and a genius crib-snatcher.

I infect anyone weak enough to let me in. I am rare in the very young and active. My victims tend to experiment with letting me in a little prior to adolescence. Unless you learn to fight me then, you are doomed for life.

If you admit to being infected by me, it only gives me more opportunity to sink in and make myself more permanently a part of your life. If you ignore the idea of my existence, chances are I will avoid you. I am often used as an excuse, but the more you use me to rationalize, the more power you give me over you.

I am not a pleasant host to carry, and I must admit that inhabiting your body is not enjoyable for me, either. But I feed off your empty space and I need it to survive.

As soon as you let me in the first time, it becomes easier and easier for me to snake my way into your life when you let your guard down. I cause your potential to trickle out of your being and your mind to waste away. You will always have the power to fight me, but I become more powerful over you every time you allow me to infect you.

Who am I?