Hi Pandita--
Anything that allows you to frighten yourself with speed (if you choose to) will help accustom you to going faster. As others have said, road biking, mountain biking, inline skating...these are all capable of pressing the "whoa" button. But so is trail running, waterskiing, kayaking or rafting, go-cart racing, and rollercoaster riding. All of these are great cross-training activities, too, which will improve your balance, strength, agility, courage, and/or tactics for skiing. Inline skating involves movements and sensations very similar to good skiing. Mountain bikes--on challenging terrain--produce similar "unexpected" imbalances and challenges to skiing, and certainly will improve strength, reflexes, focus, and aerobic condition as well. In general, if you're comfortable with your speed, push it a little faster! Adopt Dan Egan's motto to "make fast(er) normal"!
However, I'm not sure that just scaring yourself with speed is really the best answer. In fact, I don't really think you want to lose--or even decrease--your fear of speed. Fear is what keeps us alive in these sports. Show me a person with absence of fear, and I'll show you a person with presence of stupidity!
What you want is to gain confidence in your ability to perform in spite of fear. You want your fears to produce good judgment about what you can and cannot do, and how much risk you're willing to take. Helping make good decisions is the good side of fear. But once you decide to do something, you want to eliminate the negative consequences of fear--the excess tension, negative self-talk, shortness of breath, doubt, worry, and distraction that fear can produce. You want fear to sharpen your wits, not overload them--to quicken your pulse, not stop your breathing.
Right? These things come from flirting with fear, pushing your limits a little at a time--and from then looking back and "recalibrating" once you've accomplished what had once frightened you. Remind yourself that "I can do that"--as having done it is irrefutable proof! As you know, it can be an invigorating and seductive experience. There's a lot of satisfaction in looking back up at the steep terrain off Big Sky's Challenger lift and saying, "wow--I skied THAT!"...isn't there? 
Just remember to move your confidence peg up the appropriate number of notches, and update your self-image to recognize that you are a person who can, in fact, do that!
The real antidote to fear is confidence--and courage. Mark Twain said, "Courage is resistance to fear, mastery of fear - not absence of fear." Confidence comes with practice, focus, mileage, and discipline--which can very much include visualization, as Nolo points out.
Hopefully, we'll be able to get Mermer Blakeslee (author of the great book, In the Yikes Zone--a conversation with fear) back sometime as an EpicSki Academy coach. If you can find a copy of her book, grab it! Another classic book that I recommend is Inner Skiing, by Tim Gallwey and Bob Kriegel. Both of these are easy to read, with practical and intriguing advice for dealing with the many fears that can hinder--or help--us as skiers.
In the mean time, remember: you made great strides toward conquering and mastering your fear a couple weeks ago in Big Sky. Celebrate that! And look for opportunities--everywhere--to do it again.
Best regards,
Bob
