John Millar flew into a rage when he discovered that his partner was having an affair behind his back.
Text messages on her mobile phone confirmed what he had suspected for a year.
Previously he had challenged her but she just ridiculed him.
Paisley Sheriff Court was told that when the truth came out, Millar saw red.
He punched her so hard with his clenched fist that she had difficulty opening her eye due to the swelling.
When her mother saw her badly swollen face, she took her to the town's Royal Alexandra Hospital.
Police were informed what had happened and arrested him.
In court, Millar, 21, of Lammermuir Drive, Paisley, pleaded guilty to the assault charge and admitted that he had bent her fingers back and punched her as described during the episode on May 24.
It was stated that he had flown into a temper when he saw text messages.
They struggled with each other when he grabbed her hand and bent two of her fingers back.
Millar told her: "You've had me paranoid for months and you are calling me crazy.
She became fearful and he asked her to leave and pushed her from the house but followed to her car then both returned to the property again.
He said: "I think I deserve an explanation. You've ruined everything."
Millar then grabbed her hooded top, pulled her towards him and punched her on the face, causing her to fall on the bed.
She felt blood on her cheek but could not open her eye properly and was "dizzy".
Police traced the accused that night and saw his knuckles were inflamed and swollen.
Defence agent Rhona Lynch said that her client had been in a relationship with the woman for four years and he had hoped it would be a permanent one.
But, over a 12-month period he had begun to suspect there was "something going on behind his back".
Millar was made the subject of a two-year community payback order requiring him to carry out 120 hours of unpaid work and he was ordered to pay £400 in compensation to his ex-partner.