At 4 months old your dog walked beside you, because you were the safest thing on the street and their legs were short. At 11 months old, neither of those is true.
This is apparent regression. The skill did not disappear. The conditions it was trained in did.
What changed
Around the onset of adolescence, dogs respond less to a familiar caregiver's cues, and that responsiveness returns later in the stage. Add reach, strength and confidence to that, and the same leash in the same street is a different problem.
There is a plainer reason underneath it. Pulling works. A tight leash still moves your dog forward, so the habit keeps getting paid on every walk.
Check the health question first
A dog who walked well for months and then started pulling, lagging or refusing is worth a veterinary examination. Pain frequently presents as a change in behaviour rather than as an obvious limp. Check the harness fit at the same time, and whether your dog flinches when you put it on.
Tonight is not a walk
You cannot teach a loose leash while getting somewhere. Separate the two: walks for sniffing and getting out of the house, and 8 minutes on its own for the skill.
Stand still in a hallway or driveway with a 6 foot leash on a harness and 20 small treats. Wait until your dog is beside you and the leash hangs in a loose U, then feed 1 treat down by your leg.
Take 1 step forward. If the leash stays slack, say yes and feed at your leg. If it tightens, stop moving and stand still until it goes slack again.
Build to 3 steps between treats, then 5. Do 8 short sets, and finish while the leash is still loose rather than right after a pull.
Paying for 1 step feels absurdly small. It is also the version that holds, because the criteria has to be something your dog can meet 4 times out of 5.
The walks themselves, for the next 2 weeks
Use a well-fitted harness and a 6 foot leash rather than an extending one. A front clip and a back clip give you 2 places to attach it.
Give your dog 10 minutes of sniffing on a long line somewhere safe before the pavement part, so the walk is not all forward motion.
When the leash tightens, stop. Wait. The moment it goes slack, walk on. This is slow for a week, and it is the only consistent message available to you.
What the evidence says about the shortcut
Pulling is the problem people are sold a prong collar or a remote collar to solve. A review of aversive training methods found them associated with worse welfare outcomes and no training advantage. Dogs trained with aversive methods showed more stress-related behaviour than dogs trained with rewards. Reward-based methods are what the veterinary behaviour guidance recommends.
Safety note
Stop, and speak to your veterinarian or a qualified behaviour professional, if your dog barks, lunges or growls on the leash. That is not a loose leash problem, and a busy pavement is above the threshold at which any learning can happen.
What to expect
Loose leash is one of the slower adolescent skills, and it comes back unevenly. Expect the driveway to look good within a week and the high street to take considerably longer. Nobody can promise you a dog walking to heel by Friday.
Today's version, in one line: 8 minutes, 1 step at a time, on a slack leash, before you go anywhere.