James Holloway, 25, was found dead by a fellow Marine on the bridge of a Royal Navy supply ship which was moored in Dubai for maintenance work.
Post mortem examinations showed he died from a single gunshot wound to the head from his Heckler and Koch assault rifle while on the Royal Fleet Auxillary vessel, Fort Rosalie.
Colleagues of Royal Marine Holloway, who was from Billingborough, were aware he had recently split from his girlfriend, an inquest in Lincoln was told.
In a written statement Royal Marine Daniel Hilton said Royal Marine Holloway had looked at his phone when he relieved him on the bridge of the vessel shortly before 1am on October 30, 2017.
Royal Marine Hilton said he was aware Royal Marine Holloway had split from his girlfriend during the last few days and was not his normal self.
"He seemed very withdrawn, closed, keeping himself to himself, not the James we knew," Royal Marine Hilton said.
Royal Marine Hilton said he tried to engage his colleague in conversation and got the one word reply "EE" when he asked Royal Marine Holloway who his phone provider was.
"I was under the belief he was dealing with his break-up in his own way," Royal Marine Hilton added.
Royal Marine Hilton said he was aware Royal Marine Holloway had previously received a phone call from his then girlfriend in which she told him she needed an operation and would be taking her ex-boyfriend along.
This clearly had an impact on Royal Marine Holloway and he went to the gym to work out his frustrations, Royal Marine Hilton said.
One of Royal Marine Holloway's closest childhood friends told the inquest she became aware of the split and texted both his mother and two of his fellow Marines because of her concerns.
In a written statement Victoria Palframan said she had been due to visit Royal Marine Holloway in Dubai, but it was then decided his girlfriend "Holly" would go instead.
The inquest heard Royal Marine Holloway had been given shore leave to visit Holly in Dubai during the week before his death.
"On the day she left she told James the relationship wasn't working," Miss Palframan said.
She said James later texted her and told her the relationship was over.
"I knew he would be blaming himself, " Miss Palframan added.
Miss Palframan said she became concerned on October 28, 2017, after receiving texts from James.
"He was saying they trusted him with live ammunition and a rifle for 90 minutes at a time."
Miss Palframan said she was so concerned that she texted Royal Marine Holloway's mother and two of his Marine friends.
In a text message to Royal Marine Jason Hicks, Miss Palframan explained that she and his family could sometimes talk him out of a "dark place", but on this occasion it was "out of their hands."
Miss Palframan said she believed the reason her concerns were not passed on to officers on this occasion was because of the impact it would have on Royal Marine Holloway's career.
Royal Marine Holloway's medical records showed no history of mental health problems and he had not taken alcohol or drugs before his death, the inquest heard.
He last visited a doctor complaining of shoulder pain in December 2016 and had required two days bed rest after collapsing during a five mile speed march in May 2016.
The Assistant Coroner for Lincolnshire, Richard Marshall, will return a verdict into Royal Marine Holloway's death tomorrow. (Friday)
Royal Marine Holloway, who was born in Sleaford, was serving with 42 Commando based at RM Bickleigh near Plymouth.
Well wishers gathered to pay their respects to him after his body was brought back to Britain.
He had set off on the the Navy ship’s six month stint in its bid to aid allied warships in the fight against terrorism, drug trafficking, piracy and smuggling in the Middle East.
His colleagues and bosses paid tribute to his “jaw-dropping levels of fitness”, adding that he was nicknamed ‘Barrel’ because of the size of his chest.
Family members described him as a “truly extraordinary son and brother” in a tribute at his funeral at St Andrew’s Church, Billingborough, on November 30, 2017.
They added in a statement: “His smile and quick wit could raise any spirits and people warmed to him wherever he went.
"We are so proud of his achievements and his enduring determination to fulfil his life-long goal to become a Royal Marine and consistently strive for perfection.
“James was bright, funny and his amazing warmth and kindness endeared him to everyone he met.
“A loving, caring son and brother, he will be eternally missed and forever remembered.
"His loss is beyond devastating. A true gift to the world and certainly our world is a lesser place without him.”
In a eulogy released by the Royal Navy, colleagues said it was his “childhood dream” to serve his country.
He joined Commando Training Centre Royal Marines in November 2015 and was “tested” a number of times through injury.
Commanding Officer Lt Col Mark Totten added: “Energy, enthusiasm and irrepressible good humour were James Holloway’s hallmarks.
“A true professional, his desire to be the best Marine he possibly could was evident in all he did: he was quick to take notes, quick to ask questions, quick to learn new skills.
"Only recently, he was singled out as a top-performer during rural training and he relished any chance to hone his Close Quarter Battle skills.
“His approach was an example for his fellow Marines and, through his jaw-dropping levels of fitness, he set the standard.
"James sought to squeeze all he could out of his time and was never one to put his feet up – his Tough Mudder record alone showed us all that he lived the commando spirit off duty as well as on.
“Ever popular, he lifted any team he was a member of and was welcomed across the Commando. We mourn his passing now and will miss him always.”