It's highly likely you already know the answer - which is "I know my diet is probably a main factor in this".
Your diet sounds to me like itís very inflammatory, having lead to your host of gut issues, and it could very possibly be IBS. Without being a doctor able to diagnose you, nobody here can really say for sure. But you do need to speak to your doctor about it - explaining your concerns and all the symptoms youíve said here because anxiety is comorbid (happens together with) IBS in most cases. Considering that you are so afraid you might get cancer, it's better to nip this in the bud now than later so that you can enjoy a healthy sense of well-being rather than when it's no longer something you can control or it becomes intolerably worse.
If it is IBS, your doctor likely tell you what I'm telling you now, in that you need to modify your diet and reduce your stress as much as possible.
In modifying your diet, you need work out what are your triggers. I'd say you probably have blood-sugar issues too.
Anyway, the simplest way to identify the right trigger foods is to first follow an elimination diet for seven days ó a meal plan that avoids all potential offending foods, then slowly reintroduce those same foods one by one. Along the way you keep track of reactions to foods you are reintroducing in a food diary. The type of elimination diet will depend on whether you're more prone to diarrhea or to constipation. Since I don't know the answer, I'd can't offer much on what to follow.
I'd really cut out fizzy drinks and juices out, and drink water. 2-3 litres per day. Even the slightest dehydration sets off alarms in your body to make you anxious.