I actually chickened-out the first time.
I was under a lot of stress from work at the time; I had just started at a community ER and as the new guy I took a lot of flak. I started to resent medicine and actually changed my professional focus to chemistry.
Well, after working in an organic research lab for a year, I realized the day-to-day monotony of basic scientific research. I loved learning science, but disliked doing science. Things at work settled down, I found my niche, and gained the respect of my coworkers. After realizing that I really enjoyed clinical work I made a complete circle and set my sights on medical school again.
I registered for the Jan 30th MCAT this year and buckled down studying over winter break. I was nervous about my performance, but I slayed the test. I am now eagerly looking forward to applying this summer.
So, long story short, I completely chickened-out, but in the end, everything worked out fine. I think the apprehension you feel is completely healthy; just remember, you wouldn't feel this way if you were just laying around all day. A little anxiety means that you are moving in the right direction.
Good-luck!