I have been on Wellbutrin for years and recently had my dosage increased to 450/day from 300/day. This is common with users of Wellbutrin - over time, the positive effects may start to wane. Also, make sure your pharmacist / doctor is providing the same type of Wellbutrin/Budepropion with each scrip. Believe it or not, use of Wellbutrin from different manufacturers may have a differential in side effects and effectiveness for you.
Wellbutrin is stimulating and may be making your anxiety worse. It's always best to discuss these types of issues with your doctor, he needs to know when things change. We can't recommend medications, as what works for one, may not for another. It is often trial and error which is frustrating, but worth it in the end. You don't say what makes you think you have cancer, but if you have had medical testing to rule out cancer, then it's time to concentrate on "accepting" that you are fine. Worrying feeds our anxiety, ridding yourself of this worry will help your anxiety immensely. If you haven't tried therapy, I would consider doing so to help you with this "stressor." Work on enjoying your life and living for the moment, or you will just worry your life away instead of living it. You don't want to wake up one day an old person and wonder where the years went. I know it's easier said than done, and I do wish you all the best. Take care....