Learn English – Term for a person who is extremely dismissive

You could perhaps try touchy (meaning 1) (American Heritage Dictionary via the Free Dictionary):-

Tending to take offense with slight cause; oversensitive.

or perhaps hysterical (meaning 2) (Random House Dictionary via Dictionary.com):-

uncontrollably emotional.

or possibly histrionic (meaning 2) (Random House Dictionary via Dictionary.com):-

deliberately affected or self-consciously emotional; overly dramatic, in behavior or speech.

depending on the reason for the overreaction or the exact shade of meaning you are attempting to convey.

Perhaps relapser.

n. One who relapses, as into vice or error.

It is usually related with illnesses but it is also used for people who relapses back into addictions. There is a term called "chronic relapser" for these kind of addicts as well.

From Addiction Professional magazine:

Numerous failed attempts at sobriety, or a return to drugs/alcohol after a substantial period of sobriety.

While this is a simple and obvious criterion of a chronic relapser, the distinction of an individual who has attempted sobriety and failed is important.

Many addicts have a lifetime of drug and alcohol abuse but have never actually attempted to get sober. What makes chronic relapsers distinct is that they have tried and failed.