Margaret Gallagher read out a victim impact statement at Wolverhampton Crown Court before Tyrelle Harris, aged 20, was jailed for Mr Gallagher's murder.
Harris was convicted by a jury of killing Mr Gallagher, 31, at West Cross Shopping Centre on April 26 last year.
Mrs Gallagher spoke before judge Michael Chambers QC who sentenced Harris, of Helvellyn Way, West Bromwich, to serve at least 20 years in jail on Friday.
She said the loss of Karl had caused his family great heartache and left her feeling suicidal. She asked the judge to show no remorse when jailing him.
Mrs Gallagher told the court: "The first time I saw Tyrelle Harris was at Wolverhampton Magistrates Court where he was being remanded.
"He looked at me, in the direction of the public gallery, and smirked.
"I was so shocked. This has added to my grief. I am angry what this cruel individual has brought to my life.
"I have found attending the trial horrifying. I had to watch harrowing CCTV and intricate details of how his life was cut short.
"I was filled with relief when he was found guilty. Our life sentence began when Karl took his last breath on April 26, 2020, and our lives have been changed forever."
Mrs Gallagher said Karl had an unborn son who "never got to meet his father".
She described her son as being a determined person but said he was also the "joker of the family", adding: "He was always making people laugh."
Mrs Gallagher said: "He had made positive changes and wanted a bright future for his kids. He wanted to enjoy life and provide for those around him."
She said that one month prior to his murder, Karl had completed a level three in fitness and coaching.
"Just before lockdown Karl achieved his dream as a personal trainer but he never got to see out that dream," she said.
"Karl always dreamed of living in Italy but he never had the chance to visit. The only choice we have is to take his ashes to Italy."
She said she never imagined she would attend the funeral of her youngest son, adding: "This should never have happened. He had so much to live for."
She continued: "As a result of Karl's death, I struggle to live.
"I have been unable to return to work as I suffer anxiety and panic attacks.
"I have had suicidal thoughts and I am receiving trauma care for this to help me get through the pain and heartache of losing Karl."
She said Karl had a great relationship with his brother and they said they had "so much planned" together.
She continued: "Karl will never get to see his children grow up.
"Karl was a devoted dad and spent a lot of his time making memories."
Karl's parter Jasmine Pinnock also took to the witness stand and spoke before the judge. She said she first met Karl aged 17 and they became the "best and closest" friends.
Condemning Mr Gallagher's killer, she added: "Tyrelle didn't just take Karl's life that day but took what should have been mine. He took away my wedding, holidays, anniversaries and every plan in the future [we had] as a couple and family.
"[The defendant] showed no remorse for his actions. First, the heartbreaking trial and he told lies about Karl to save himself. Tyrelle took away a great dad, a loving son, a great brother and loving best friend."