Miss Trojniak, 29, an assistant manager at Nando’s in Collier Wood, south London, was planning to leave him after she had grown close to a colleague.
She had contacted the Post Office to redirect her mail to a colleague’s address where she was planning to stay after leaving Sroka.
But he found a letter confirming the redirection, which was sent to the home they shared in Croydon, south London, sparking a row.
He stabbed her six times, and slashed her once, before turning the knife on himself then calling 999 to tell them his girlfriend had been stabbed.
Sroka made a swift getaway from the scene in his car, and was later found crying for help at about 10.30pm.
He admitted manslaughter, citing loss of control, but was convicted of murder at the Old Bailey.
Sroka was jailed for life with a minimum of 23 years.
Sentencing Sroka, Judge Mark Lucraft QC said: “It is clear the relationship was coming to its end.
“Joanna had become disillusioned and wanted out.
“She feared the reaction she would get. She felt she had to plan in secret for a new life away from you, and the control you were exercising over her and her life.
“She had expressed the thought that you would kill her if she left you. Sadly her fear turned out to be true.
“By your actions, you took away her life as she was about to embark on a new chapter.
“This attack was a culmination of a relationship in which you had clearly demonstrated jealousy and a desire to control.
“This was, in my view, a premeditated attack on Joanna.
“Having heard of her plan to leave and have her post redirected, you planned your move.
“The attack was brutal and prolonged. The injuries you caused were horrific.
“You then left her for dead and showed little, if any, genuine concern as to whether she was alive.”
The couple, who were both Polish nationals, had been in a relationship for six years.
A friend of the victim’s had advised her to end the relationship with Sroka, but she said she had lent Sroka £5,000 which she would not get back.
When asked why she did not just walk out and leave, her response was “are you crazy, he will kill me”.
On the night of her murder, Miss Trojniak had exchanged messages with a friend saying she intended to talk with Sroka that evening about their relationship.
While she was at work, Sroka opened the Post Office letter, and bombarded her with furious text and Whatsapp messages.
At the same time, she text her colleague saying: “I am going home now, I need to talk, no other choice. Don’t know what going to happen and how it’s going to finish today”.
She arrived home about 7.30pm that evening, and just over an hour later, she was dead.
In an impact statement read to court, her mother Stanislawa Trojniak said: “My life ended on 24 March 2016 when I received that awful message that my beloved child was dead. It was like if somebody punched me straight into my heart.
“I would give my own life so Joanna could live. She was such a happy child and she gave so much love to everybody who knew her and loved her.”
Miss Trojniak was planning to go home in May 2016 for the first communion celebration of her ten-year-old god daughter, she said.
Her mother added: “I wish every parent to have a child like her.
“She was a very good person and she believed in people and that it is worth helping others and that you can change anyone, but it is not always possible.
“That’s why she is not with us anymore, and I blame myself for her death.
“It was my fault because I did not warn her about this evil, which was predictable.
“For me everything died together with my beloved child.
“My day starts and finishes at the cemetery.
“Now it is the only place where i feel at peace and safe. I know that nobody will ever hurt my child again.”
The victim’s father died in 2014, and Miss Trojniak helped support her family in Poland from her earnings in the UK.
Her mother added: “She never said anything about the man she was living with. I felt she was scared to talk about him or was avoiding the subject.
“I met him in 2014 when he came to my husband’s funeral. I did not like him but I did not say anything to Joanna as I did not want to upset her.”
Sroka has previous convictions in Poland for assaulting a family friend in 2007, and from 2005 for drink driving.
Di Middleton QC, in mitigation, said: “What is clear is that, notwithstanding the fact that this is a murder, this defendant did feel immediate remorse, and demonstrated that remorse – remorse for the killing – by wanting to end his own life.”