First of all, failing isn't "inevitable". It just feels like it is. I've made similar grim predictions (indeed, on an emotional level I almost always believe that things will either stay the same of get worse), but many of my "prophecies" turned out to be wrong.

There is a reason why you are having difficulties with your job, and you've named it: poor concentration. Now, avoiding failure is as simple as doing all that you can to alleviate your attention problems. Perhaps your pdoc can add a stimulant like ritalin or concerta to your meds regimen that might help your concentration? Also, if you don't already have one a healthy lifestyle, especially good sleep habits and exercise, can help. You might also want to start meditating.

Good luck! Distraction is a hard dragon to slay (it still rules my life to a much greater extent than I care to admit). But I think you can overcome this!