What am I doing wrong?
Pretty much everything!
Do I need to change my technique??
YES!!! Sorry, I'm not trying to beat up on you, I just want to stress that when you're teaching your dog to walk nicely on a loose leash you pretty much have to abandon the idea of simply "going for a walk". Every second you're out there with him you're training him - and that can either be actively teaching him to do it the way you want him to, or teaching him that he doesn't have to. Your techniques seem to be accomplishing the latter, not the former. Think about it from his point of view - what he's doing is working for him! He still gets to go where he wants to go when he pulls, so what incentive does he have to change what he's doing?
He wants to be ahead so he pulls, and you shorten the leash. How is that teaching him not to pull?
You walk a few steps and make him sit. He gets rewarded for sitting. What does he get for walking nicely and not pulling?
If you have to practically sit on his butt to get him to sit he either doesn't fully understand the sit command (he may not have generalized it to all circumstances and situations), or you haven't worked on it in a low distraction environment until it's solid and then gradually increased the level of distraction and difficulty, so you're asking him for more than he's capable of doing (an exercise in futility), or you haven't reinforced focus and attention enough so there's no foundation to build on once you increase distractions, or he simply doesn't care what you want him to do because he doesn't have to - there are no consequences, good or bad, for compliance or non-compliance. Or, all of the above!
I'm not sure how long you've had him, but if you're working on getting him to lay down on command for 10 or 12 minutes a day (just once a day?) and he's already 9 or 10 months old and still won't always do it, then you need to re-evaluate everything.
For one thing, I'd be doing LOTS of short training sessions every day. If you can do 5 or 10 minutes 3 or more times a day, that would be great. How exactly are you working on teaching him sit and down? Where do you train him (in the house - what room/s? in the yard?)?
We have lots and lots of threads about teaching loose leash walking, have you read any of them? I can find some for you if you haven't seen them yet - they have lots of links to techniques that WILL work if you're committed to trying them and sticking with it.