I've had a very similar problem since I was 15 years old (I'm now 24). I always thought the problem was with my body, but only in the last 3-4 years have I started to realize that the problem is all in my mind. I'm not cured of it, but I will share with you what has helped me.
1. Find out which issues scare/stress you out. Make a list. Break the problems down into manageable steps... and start DOING. Procrastinating on life is a major source of stress and fear.
2. Don't obsess about your stomach problems. We mistakenly think that just because we are constantly thinking about our stomach problems, that means we are dealing with it.. Wrong! Obsessing over our stomach problems just causes more anxiety. CLEAR YOUR MIND.
3. Do productive things that focus your attention elsewhere... like a jigsaw puzzle, study, walking on a treadmill.. etc Doing productive things will raise your self-esteem and focus your attention elsewhere.
4. BE HAPPY. When I'm happy and having a good time, my stomach no longer bugs me.
Summary...
CLEAR YOUR MIND. BE PRODUCTIVE. BE HAPPY. BE CONSISTENT.