Your pup isn't a pup any more and you have at points though "what have i done"?
Teenage dogs (dogs between 6 and 18 months) just like human teenagers can be hard work. They are still learning about them selves and their environments but because they are fully grown we forget they haven't been on the earth that long.
Walks can hard with lots of pulling, failed recalls, randomly eating of "stuff", not greeting humans or dogs in a manner in which you would like. It can be very hard and this is when most dogs get rehomed. I can guarantee they will embarrass you with a move you haven't seen before at the worse timing.
But if you just hang in there and put the time in you will come out with a dog you wouldn't be without in the end.
We can get in the habit of disengaging ourselves but if we make sure that we fully focus on that dog walk and do lots of games for engagement, our dogs will learn to focus for longer, they will want to hang out with us more and they will stop looking for that engagement elsewhere so much.
The easiest way to get engagement is to start it from the get go. Equipment on and then train before you step out the door. Then just outside the door. The more they engage the quicker they will get to the smells they want.
Simple and easy games, are great. But to start, don't make it too hard, set expectations low. Even if its a half look then take it and Reward it. Set an easy achievement level, don't make it too hard. Ask too much and thats where the frustration sets in.
Often a sit at the wrong moment is just too hard for the dog to do.
We are aiming to change the dogs mind set to one of "training / focus mode" when out. Mixed with ok and free time to sniff and then back to focus. Slow increase the length of focus mode. But to start the more free mode you give the dog the harder it is to get them back. 1minute on, 2 minute off sort of process. You can change where the lead is attached on the harness to help set which mode for you and the dog to remain consistent.
Walks need to be 85% training time. Or out of 14 walks a week (for example) 10 are more focused on training and are possibly shorter. Swap walks for training in the garden are fine too. A 20 minute training walk will get you further to your goal and will tire the dog out just as much as an hour hike. Do this for 2 months (so its not forever) and you will reap the benefits later on.
Other Dogs are mostly young dogs motivation, they are fun and they are still understanding themselves compared to others. Main rules of engagement do have to be taught that yes you can see that dog but not if you drag me to it. When on bigger walks the rule of 1 in 3 might be good. Play with one, only a quick sniff another and not engage with the third.
To slow your dog down or calm the excitement it is a level of impulse control needed.
Eye Contact is a great way to build in as a go too behaviour when seeing another dog.
This will be the key to your heel work.
All heel work is subject to the level in which your dog has learnt to hold it so far. Level 1 in the house to level 10 outside past another playing dog. Slowly you can increase your level by using duration and distance for practice.
Training can be counted in reps (rewards) how fun and engaged you can be with your dog outside matter. This is a great way to measure your improvement. If your dog can do 10 reps of a simple game near another dog and you know you are getting there.
Work as fast as your dog does. Quick thrown food, easy play and energy from you will help get their attention especially when working with near dogs then you can slow it down as the dog learns more.
Tight leads, pulling away and lots of no's will only make them more frustrated and thats when the frustrated greeter can be created. If you always hold your dog in a heel it will expect you to always do that.
Toy play is even better, even if for short bursts. Play with your dog outside, they will enjoy being with you more , simple.
To get you through think:
Slow and Steady
Create good habits
Most of all: Patients!