Benjamin Field, 28, admits he "gaslighted" Ann Moore-Martin, 83, as part of a plot to benefit from her will after her death.
He had declared his love for the retired head teacher in a series of letters, postcards and poems during a year-long seduction - even though she was 57 years his senior.
Field, the son of a Baptist minister, denies conspiring with his magician friend Martyn Smith, 32, to murder Miss Moore-Martin.
Prosecutors allege Field targeted the deeply religious spinster a few months after allegedly murdering her neighbour Peter Farquhar, 69.
Field, an English literature student at the University of Buckingham, also left messages written on mirrors in her home purporting to be from God, as part of his gaslighting campaign.
Oxford Crown Court heard that Miss Moore-Martin was admitted to hospital in February 2017, having suffered a seizure.
The following month she was readmitted to hospital after suffering a further seizure. In May she died from natural causes following a stroke.
Miss Moore-Martin changed her will to leave her home in Maids Moreton, Buckinghamshire to Field, but changed her mind shortly before she died and alleged Field had poisoned her by giving her "white powder".
David Jeremy QC, defending Field, asked him: "Were you the cause of her illnesses?"
Field, who was giving evidence for a sixth day, replied: "No, no way at all."
Mr Jeremy asked: "Did you administer drugs or powders to her?"
The defendant replied: "Never, no."
Field told the jury that he had deceived Miss Moore-Martin during their relationship.
"She was the wronged party. I was very much in the wrong," he added.
Field said he was not surprised to learn that his elderly lover had cut him out of her will shortly before she died.
"It seems that my influence upon Ann would be discovered and (the will) changed because I was not in her life influencing her and (our relationship) was all based on lies," he said.
"It was over. I was saddened by that. I thought she would be upset. My plan for myself is to try and succeed legitimately."
Field described losing out on inheriting Miss Moore-Martin's home as a loss of a "massive potential gain".
"But it was something that had caused some inner conflict earlier in the year and I was to an extent pleased to move away from a devious life and had an end to it," he said.
Field has admitted defrauding Miss Moore-Martin of £4,000 to buy a car, and £27,000 to buy a dialysis machine, by claiming his younger brother, Tom, 24, was seriously ill.
Field and Smith, a magician, deny charges of murder, conspiracy to murder and possession of an article for use in fraud.
Field, of Wellingborough Road, Olney, Buckinghamshire, also denies an alternative charge of attempted murder. But he has admitted four charges of fraud and two of burglary.
In addition, Smith, of Penhalvean, Redruth, Cornwall, denies two charges of fraud and one of burglary.
Tom Field, also of Wellingborough Road, Olney, Buckinghamshire denies a single charge of fraud.
The trial was adjourned until Wednesday.
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