Dean Bird, aged 29, from Victoria Avenue, Bloxwich, threatened postmaster Jarnail Singh with a knife at Blakenhall Post Office.
During the struggle, Bird caught Mr Singh on the index finger with the blade, causing him injury.
He was jailed for four years and sentenced to an additional four years on extended licence after admitting attempted robbery and possession of an offensive weapon at Wolverhampton Crown Court.
Sentencing Bird, Judge Nicholas Webb, said: "You were watching a political programme on television and you became angry.
"You went round to the nearby post office, where you were known as a regular customer.
"You produced the knife and demanded money.
"I am quite certain that you are a significant risk to the public. You have a mental and behavioural disorder which you don't seem to be able to control."
Giving evidence, Bird was asked by his barrister Oliver Woolhouse: "Is it correct that you have mental health issues?"
He replied: "So I'm told.
"I wasn't taking my medication at the time, I probably hadn't been taking it for two, maybe four weeks.
"It had been building up, I had been getting worse and worse.
"I just felt I needed to do something.
"I felt angry at what I was seeing on the television and the messages they were giving me.
"I had been watching a number of programmes, on the politics channel.
"I was arguing with myself and shouting at the television.
"Then I decided to do something stupid.
"The post office is about a two or three minute walk from where I live.
"I've always got on with the bloke who runs it, it wasn't personal.
"I've was just there to do something to make a statement.
"Afterwards I rolled a fag and was on my way to the police station to hand myself in when I saw an officer."
In mitigation, Mr Woolhouse said: "He has mental health issues that he doesn't see or accept.
"When he takes his medication he doesn't show any signs of being a risk to the public.
"He took the knife with the intention of brandishing it, rather actually using it to inflict harm.
"Though I wouldn't seek to blame him for doing so, the injury was inflicted when Mr Singh tried to grab the knife.
"He admitted what he had done to the police straight away."