I have had a similar issue. I then start to try and respond just as quickly. I can play a G/30 and after I get mated I see that I have used a grand total of 3 or 5 minutes.
One thing I try and do is play with a board and make my moves on the board then on the computer, then after the computer makes the move I move the pieces on the board. It makes me slow down some, but not a lot.
Still struggling with this.
I've always had a problem when playing chess against a computer that's kept me from learning much and developing my game. No matter how "strong" or "weak" the computer's settings are, its reply to any move of mine is all but instantaneous--fractions of a second.
I fully understand how ridiculous it sounds, but this blink-of-an-eye response never fails to psych me out, every single time. Having agonized for minutes over each move, I feel irretrievably unbalanced by an adversary that can respond so quickly, and with such (however illusionary) ease. Half the time, I'm too rattled to finish the game; the other half, I'm too rattled to play decently--to this day, I still haven't beaten a computer opponent.
Has anyone else experienced this peculiar mental stumbling block--and if so, has anyone discovered ways of overcoming it?