In a column for England's Daily Telegraph, Harmison, who retired from cricket last month, said Trott should take professional help for permanent solution to his problems.
"There are a few of us in the game he can speak to, but if he never plays international cricket again and is happy, that is a better outcome than playing and suffering," said Harmison.
"Hopefully he will be back, but the only person who can sort it out is Jonathan."
Harmison, who made his England debut in 2002, took 226 wickets in 63 tests, including a memorable seven for 12 against the West Indies in Jamaica in 2004 which briefly earned him the top spot in the test bowler rankings.
Trott's struggle reminded Harmison of his own battle against depression that twice put him on the verge of withdrawing from the squad.
"Nobody knows more than me what it is like to be depressed while you are on a cricket tour because I spent 10 years hiding it as homesickness," said Harmison.
"I said I was homesick and that was actually used as a stick to beat me with. It was not just homesickness, although that did not help.
"It is a chemical imbalance in the brain and it is something I battled with for years. It tended to be inflamed when I was away from home because I did miss people, I was lonely and I did not have my support network around me."
England opener Marcus Trescothick pulled out of England's 2006-07 Ashes Tour with depression and never played international cricket again while spinner Michael Yardy also departed from England's 2011 World Cup team with depression.
Harmison, 35, wished he had opened up more about the problem during his playing days but said he was scared of the reaction that would follow.
"It is so, so tough. I can remember sleepless nights in hotel rooms where I would be in tears and then going out to play the next day," he said, adding he was more vulnerable to depression when alone.
"I used to surround myself with people. I took a dart board with me so that people would come to my room after training or a day's play.
"I was comfortable with people around, it was when I was alone that the world caved in."
(Writing by Sudipto Ganguly; editing by Amlan Chakraborty)
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