Christopher Cole, 31, of Clun Road, Wick, is standing trial over the murder of Sarah Clayton, 21, who was found dead at the Buckle campsite in Seaford in May 2018.
Mr Cole told Lewes Crown Court he was frantic after returning to the tent to find Ms Clayton cold and unresponsive.
A jury heard the couple had been arguing and drinking during a weekend stay at the seaside campsite.
Mr Cole told the court his family had taken Ms Clayton in when she was using drugs and living in a tent.
He said she was given strict rules about staying away from drugs or she would be thrown out of his parents house in Littlehampton.
Mr Cole said they had been in a relationship for more than five months before her death and planned to marry.
The were on a weekend trip to the campsite at Seaford, East Sussex, in May 2018.
They argued after coming back to find the tent waterlogged, Mr Cole agreed.
Other campers described hearing choking noises coming from their tent overnight.
A woman sleeping in the next tent described hearing a man saying: “If you don’t shut up, I will shut you up.”
Mr Cole said he went back to the tent around 6am on Sunday morning after waking up to go to the toilet.
“I had a gut feeling there was something really not right here,” Mr Cole said.
“I knelt down next to her and shook her but got no response at all.
“She was very stiff.
“I knew from experience, while the paramedics were coming I should try something.
“I tried my best.
“At this point I was frantic.
“It was somebody I really cared about.
“It was emotional overload.
“You imagine as if that was your partner.
“It just seemed to go on and on like I was on an endless nightmare.
“I was hysterical.”
Duncan Atkinson QC probed Mr Cole over his refusal to answer police questions when he was interviewed later.
“You had been arrested on suspicion of murder,” Mr Atkinson said.
Mr Cole said: “I can’t explain why it ended like it ended.”
Earlier, the court heard another ex-girlfriend had accused Mr Cole of grabbing her by the neck.
Mr Atkinson said: “You had become angry with Sarah and had sought to exert control over her by restricting her ability to breathe, as you had done in past.”
Mr Cole said: “I disagree with you totally.”
Mr Atkinson said: “When daylight came, you realised what you had done and by that Sunday evening you were seeking to protect yourself from what you had done, to hold back answers until you could think of better ones.” Mr Cole replied: “I don’t agree with you.
“I’ve got a mind of my own.
“No one can lead me.
“Everyone makes their own choices in life.”
Mr Cole denies murder. The trial continues.