Mark Restivo, 39, had dreamed of being on the NYPD since he was 5 years old. But when he was injured on the job and made to retire after only a few years on the force, an old habit with painkillers spun out of control. In just a month’s time, Restivo went from being an officer to an opioid drug dealer. Now a drug counselor himself, he tells The Post’s Hannah Sparks his story of addiction and recovery.

It was August 2011, just after Hurricane Irene tore through the city. I was going 100 miles per hour in my car down the Northern State Parkway, dodging wood debris and down power lines. At one point, I had to slam on my brakes to avoid a fallen tree.

I’d spent the storm holed up with my unit at the Transit District 32 in Crown Heights, and I hadn’t seen my wife and two kids, in Levittown, LI, for more than 30 hours. But I wasn’t thinking about them. I was headed straight for the pharmacy.

I was just 22 when I took my first painkiller, in 2002, after getting a root canal. The dentist warned I might be in pain, and so he prescribed me 20 Vicodin, just in case. I took them as directed.

They made me feel different. Not how you might think. Yes, they numbed the pain, but they also made me feel like I could go full throttle, like I was shot out of a cannon. I liked it. Back then, the pills were so common — it seemed like everybody had them, and doctors just gave them out. When my supply ran out, it was easy to get a refill.

I married my wife in 2007, and one year later, at 28, I enrolled at the New York City Police Academy. By that time, I had built up a tolerance: Every day, I was taking anywhere from 50 to 70 Vicodins, 10 milligrams each — not to get high, but just keep from feeling sick.

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I knew I wouldn’t be able to sneak that many pills during training. So, in January 2008, I stopped, cold turkey — going through all the nausea, headaches, body aches and shivering of withdrawal. It felt horrible, like the flu on steroids. I don’t know if it was the hustle of training or being told where to go and what to do all the time, but somehow I got through it — without any extra help.

As a rookie on the force, I was able to put down the stuff for a while. I even had my first kid, a daughter, while I was clean. But I still obsessed about the pills all the time. I knew that one day, I’d have to go to the doctor or dentist, and they’ll give me a legal prescription. “I’ll be fine,” I told myself. “I can take these again and be OK.”

I finally got my wish in December 2009, just three months after my daughter was born and about a year after earning my badge. While patrolling the Utica Avenue subway station in Crown Heights, I saw a couple guys jump the turnstile after surrounding a woman. They seemed to be harassing her for her purse. When I identified myself as a police officer, they attacked me, shoving me down a flight of stairs. I ended up in the hospital with a couple herniated disks and my meniscus torn in two places.

Before referring me to an orthopedist, the ER doctor handed me a script for just 10 Vicodins.

On crutches, I hobbled across the street to the 24-hour pharmacy. I was heading back to command, so I took two or three pills for the pain. By the time I got home after my shift, the bottle was empty.

I ended up having three surgeries on my knee between the end of 2009 and 2010. My prescriptions went from these big Vicodin pills to these smaller, even more potent pills. All I had to do was mention pain in my knees or my back, where I’d been injured, and the doctors I saw — and there were many — would prescribe, prescribe, prescribe. By the time my second kid, a boy, was born in July 2011, I was taking 30 to 40 doses a day of slow-release OxyContin and fast-acting Roxicodone, in the highest milligrams available.

As an officer, I had been on the 20-year track. But I had to retire in the summer of 2012 because of my injuries. Once you can’t perform all aspects of the job, it doesn’t make sense to keep you around.

My addiction only ramped up in retirement — and within a month, I was a full-time drug dealer to support my habit.

Doctor shopping, I learned that doctors’ waiting rooms were like drug flea markets. I’d approach patients and ask what they did with their painkillers — if they usually took them all, if they’d be willing to sell. Sometimes I’d offer to sell the pills, which went for about a dollar per milligram, on their behalf and give them a cut of the profits. I’d use the money to buy more pills.

Being an ex-cop made it easier. If there was ever any question, I’d just flash my retired NYPD ID, and doctors and pharmacists would supply. And my training taught me how to avoid being arrested: I would arrive at a drug deal early, and drive around the area to make sure no one, especially undercover cops, were nearby. I would also use my law enforcement status to scare the people I was selling to, in case they were thinking about screwing me over.

I may have been fooling my doctors and pharmacists, but I couldn’t hide from my wife. I was on my cellphone all the time. Money was disappearing from our bank accounts. Sometimes I’d tell her I was going out for the newspaper, then return home five hours later, close to midnight, after stops at the 7-Eleven parking lot or in the cabinetry aisle at the local Home Depot, where I’d drop the drugs in a drawer and wait for my customer to pick it up and leave cash. One time I met a guy there and we literally stood in the middle of the aisle and counted pills.

My wife — now ex — saved my life when she gave me an ultimatum: I agreed to take a urine test, and if I failed I’d go to rehab.

I had a plan. I took the container to the bathroom that my daughter, who was almost 4 years old at the time, had just used. I dipped the cup into the commode, and accidentally scooped a piece of toilet paper, so I reached my finger into the cup and pulled it out — forgetting I had just used that same finger to crush up an Oxy. My daughter’s urine came back positive for narcotics.

In September 2013, I went to the Mirmont Treatment Center in Media, Penn., and enrolled in a program designed specifically for first responders. It lasted 28 days.

There, I finally got the nerve to say it out loud for the first time: “My name is Mark, and I’m an addict.” I had to pause. I finally felt understood.

As the month came to an end, I was sure I never wanted to take another drug again — not even Motrin. But my wife confessed she didn’t trust me, and our psychologist suggested once-monthly Vivitrol, a nonnarcotic injectable drug, “if you ever want to see your kids again.” So I tried it. It kept my craving at bay for the next 18 months, while I continued with outpatient rehab and found a new doctor — a real badass, cowboy type, who didn’t take s - - t and made me give him my sponsor’s number.

In March 2014, my now-ex and I had our third child. The following September, just a day after my 2-years-sober date, I was hired as a drug counselor at Phoenix House in Wainscott, LI. The work reminds me I’m not different from my clients, that I could be right back where they are at any moment.

If life were fair, I would be dead. But I got a third, fourth, fifth chance at this. Since then, I’ve been blessed with a fourth child, and am now engaged.

I’m still a baby in recovery, and struggle with it every day. But I know long-term sobriety is possible. I’ve seen it. And I’m looking forward to that.