Doctors have described the "heartbreak" of seeing patients being treated in corridors and 14-hour ambulance wait times become "the norm".
Staff morale is at "rock bottom", with many thinking about leaving the profession, a doctors' union said.
There is "no way that anybody could continue working like this", consultant Nicky Leopold said.
The Welsh government said it was working on a plan to deal with the pressures.
BMA Cymru chairwoman Dr Iona Collins said: "Ever since the NHS has been in existence we have always been, what I feel is now in hindsight, crying wolf.
"We always say 'we don't have enough staff, we don't have enough resources, patients are coming to harm' and this is what we hear, year after year.
"Now, as far as I see this, the wolf is here. I don't see how things can get any worse."
Ms Leopold, 42, a consultant geriatrician from Swansea, has started to question her future in the NHS.
"Sometimes I just think: 'Can I carry on doing this?' In its current form, there's no way that anybody could continue working like this.
"We're trying to find innovation and efficiencies every single day, but I can't see where there is more efficiency to be had.
"All around me staff are leaving, staff are crying... many people are already voting with their feet and leaving. But of course that just strips the service even barer."
She said her elderly patients faced long and "extremely distressing" waits for admission to hospital.
Those "horrible" experiences, she said, were a regular occurrence for people with chronic conditions or cancer, leaving some afraid of having to call an ambulance.
One of the biggest barriers to timely treatment, she said, was the inability of hospitals to discharge patients because of a lack of social care options.
Those in hospital longer than a week lose their existing care package, a situation she called a "truly heartbreaking".
"If it took them eight or nine days to get better and they could now go home, they go to the back of the queue to wait for care again. And that can be several months."
Consultant urologist Tim Appanna, 52, said staff shortages in everything from radiography to diagnostics had an effect on timely treatments.
"We're literally just able to see cancer patients and urgent patients," he said. "We're not able to see much in the way of routines at all.
"Patients who are on routine lists are going to be waiting a very long period of time before they get seen. And if they need a procedure, they'll be waiting even longer.
"I'm in this because I want to do the best for my patients, but it's unbelievably frustrating when I know I could do a better job."
One member of staff in the NHS, who asked to remain anonymous, said added pressure during the busy winter period would mean having to put two patients in cubicles meant for one.
They added: "Waiting for 12-14 hours in the emergency department is now the norm. Practices such as using corridors, which was stopped during the first waves of Covid, unfortunately has now restarted.
"There is no room to see patients and those diagnosed with significant illnesses and requiring emergency surgery are sat waiting in waiting rooms or chairs."
Last month it emerged NHS leaders in Scotland had discussed whether a "two-tier" health service was an option, with the wealthy paying for treatment.
Mr Appanna said, while unappealing, it "may become an inevitable thing".
"I am a valleys boy, I was born and brought up in Wales and I'm heavily invested in the NHS but think we've got to make decisions.
"If we can't actually deliver health care for our patients, then we have to look at what else we can do."
Dr Collins said workforce numbers might suggest all is well, but many staff are no longer working full time, with some realising their own health and livelihoods were being harmed.
"It's little wonder that as a consequence doctors are having to look at themselves and unfortunately, with guilt, they leave."
"There is no exaggeration. There is no pathos here. This is the reality of the here and now," she said, adding there was "no shadow of a doubt" that this was the worst she had seen.
"I would say that now, the people we have are left in the NHS now are the idealists who really believe in the principles of the NHS of delivering treatment on the basis of clinical priority and free at the point of care," Dr Collins said.
"And we absolutely want to stick to those basic principles but I'm afraid that the doctors and nurses who are in there who have the choice to work elsewhere but they stay in the NHS to try and keep this system afloat.
"They are less and less able to justify to themselves why they should remain in that NHS system, because it is hurting them."
She also warned that too few issues were being captured correctly, such as a lack of privacy for patients or no patient dignity, or people being treated in ambulances which are stuck outside A&E.
The reporting system currently in place, which is not anonymous, was not being used to improve patient safety, she added.
The Welsh government said a short-term plan to deal with current pressures on NHS staff was being developed in addition to its existing workforce strategy.
A spokesperson added: "We greatly value their tireless commitment to serving the public and understand their frustrations.
"There are more staff working in NHS Wales than ever before and this year we are investing record levels in training and professional education, £262m, including more training places than ever before."