How do I not play too fast when playing the computer? When I'm playing against a computer, it moves virtually instantly. Automatically, I'm speeding up as if trying to match it. Obviously, I'm not a computer, so I make horrible blunders if I move so quickly. When playing OTB against a live player, I take considerably longer for my moves (I'm fine with playing a three hour game) and consequently I play a lot better OTB. By the way, I virtually always play without a clock.

Is this common? Are there any tips besides "just don't do this"?
This is a very bad habit you are getting in to. Of course it doesn't really matter if you lose to the computer in this situation but you will face something similar in OTB play with clocks. 

If you have 10 minutes left to finish the game and your opponent has less than 2 minutes then this is a very big advantage for you. If you are not playing with increments then your opponent will have to start blitzing his moves out to try and avoid losing on time. He is much more likely to make a mistake. If you copy him and start blitzing too then your advantage disappears.

You should try and have a set routine which you go through regardless of the opponent's speed of move, some questions you automatically ask yourself. Why did my opponent play his last move? What is he threatening? How does it stop my plans? Does it give me some new possibilities?

If you consider these kind of questions it will slow down your play and help you to stop blitzing unnecessarily.