Exacerbating her grief is the question of whether Parker might still be alive had police not refused to send an officer to check on him when he took a toxic cocktail of drugs. West Mercia police, which twice rejected calls for help, has admitted it failed to discharge its "duty of care" and should have attended.
* Whistleblower warned police against axing mental health callouts
The force's call handlers had cited Right Care Right Person (RCRP), a controversial scheme under which police are refusing to routinely attend mental health callouts. Chief constables claim it has freed up more time for crime solving.
However, The Times has exposed a series of deaths after forces used RCRP to deny deployment, even when cases met the threshold of "immediate risk to life" under which police attendance is a legal requirement. Charities have warned that highly vulnerable people have been effectively abandoned by the state because health services have not filled the gap left by police.
When Parker texted his mother on October 8 last year to say "goodbye" after detailing the doses of prescription drugs he was planning to take, she knew the threat was real. Parker had a history of mental health problems, including psychosis, and also suffered serious health symptoms, related to an earlier illness, which made his life difficult. But he had never attempted suicide before.
Mullen lived 90 minutes away and did not have a key. "I was really panicking," she said. "I knew they needed to break that door down."
Her experience mirrors that of other bereaved families who have raised concerns about RCRP, including being bounced between emergency services, leading to delays in attendance.
She called police and was told "we don't do mental health calls". West Midlands Ambulance Service told police they could not immediately attend due to 200 other waiting patients, and logged the call as category three. As per procedure it was upgraded to category two after 40 minutes.
"If they had logged it as a category one, and I have seen that some other areas do with drug overdoses, then they would have gone out within ten minutes ..." Mullen's voice trails off as she thinks about what might have been.
An ambulance crew arrived at Parker's address, in Worcester, about one hour after Mullen's first call. At the same time he sent a final text to his mother asking her to tell the rest of his family how much he loved them.
"So he was still conscious at that point, and help was outside," Mullen said.
But paramedics were unable to gain entry, and asked police for help breaking in. West Mercia police refused deployment a second time, and the call was transferred to the fire service. Fire officers finally gained access more than two hours after the initial 999 call, and found Parker inside. He was still breathing but unresponsive.
At the hospital, doctors worked for several hours to try and save Parker, but it was in vain. Mullen and other family members sat with him while the life support machine ran down. She recalled: "I asked the doctor if, had he been brought in earlier, could they have saved him? He couldn't say."
She had enjoyed Sunday lunch with her son just a few days before. "He had his struggles, but he was doing OK. The fact he told me what he was going to take [to end his life] makes me think he wanted to be saved and this was a cry for help. He was not only my son, he was my best friend."
At an inquest last month David Reid, the senior coroner for the West Midlands, recorded the cause of death as a drug overdose but concluded it was not possible to say whether Parker intended to kill himself.
DC Lewis Cleaves, from West Mercia's professional standards department, told the inquest at Worcestershire coroner's court that an internal investigation found officers should have attended.
He said: "We were clearly told that Christian intended to take his own life. We should have attended and consulted an ambulance from the scene with his condition."
Reid welcomed the police's formal admission of a failure to send an officer.
Afterwards Mullen felt comforted by the reassurance that West Mercia would learn lessons from her son's death and use it as a case study to demonstrate, as Cleaves said, that "RCRP is not a one-size-fits-all".
She said: "I wanted recognition that they got it wrong because on that day I felt absolutely helpless. It doesn't bring him back, but it might help someone else."
Her comfort has since turned to further distress, however, because West Mercia's corporate position appeared to contradict what Cleaves told the inquest.
Contacted for comment, the force initially claimed that its internal review had determined that the ambulance service was the most appropriate agency to respond and that "during the inquest, the evidence from police was that police could have attended, not should have".
Challenged about the accuracy of this claim, West Mercia gave a second statement. The force conceded that its call handlers could have "acted with compassion" and had received "reflective practice", the lowest form of censure. However, it insisted that RCRP was followed correctly, that it was right to defer to the ambulance service, and did not accept at the inquest its officers should have attended.
The force's press office suggested that a reporter from The Times had misheard. The Times obtained the official written record of the inquest. The coroner clearly stated West Mercia police's acceptance that it "failed properly to discharge its duty of care towards Mr Parker by ensuring that a police officer attended his address when its staff were first made aware of his likely overdose".
West Mercia has not responded to follow-up questions about why its corporate statements directly contradicted an officer's evidence to the inquest, as well as a coroner's conclusions.
Mullen said: "I have real concerns that they may have just been saying the 'right' things at the inquest but are not actually making any changes."