Echo what the others have said. I'd just add that you can show leadership- or absence of leadership - in every single interaction. My daughters pony Oscar will become sillier and spookier when ridden if my daughter relaxes her expectations of his manners on the ground. You need to be clearly in charge all the time. And by that I don't mean be rough or hard but be the one who controls all 4 feet all the time. Eg backing away politely from the door if you go to put hay or feed in the stable, waiting politely on a loose lead rope if you stop to chat to a friend while turning out, yielding away if you open a gate and need him to step back and across etc etc etc. Horses are far happier and more relaxed when they feel they can turn decision making over to a consistent and fair leader. On the other hand if they don't have faith in your leadership they feel they are responsible for their own safety and will take matters into their own hands. it's not naughtiness its self preservation.
This is exactly why so many folk comment that a horse is fantastic for the likes of the farrier and the odd friend or riding instructor who pops in, does the tiniest bit of work with them and it makes it seem so easy but in reality it's easy to be firm and disciplined with a horse for an hour, but when it's every single interaction that forms that respect, you just can't afford to let them take an inch. It's definitely why so many of us horse owners have issues as there's too much emotion wrapped up in it, where as these riding instructors Farriers and vets can seem like they've got the knack. They're not so emotionally invested!
Glad to hear you're determined suzy, patience is key and it sounds like you've got the balls to give it a go. I get really frustrated when folk who've lost their confidence completely blame the horse and can't see that they have some work to do themselves. I learnt that the hard way as I had that immediate reaction myself but getting help is fab (although don't defer to them completely, as you might inadvertently pass over control to that help and the horse will then see them as a leader but not you!)
Lunging was beyond us as well, I still can't lunge my boy for peanuts. I've been shown heaps of times but my boy is so good at reading me and telling me I'm doing it wrong. I gave up TBH! There's some suggestions about a similar thing to join up, and I used to do it quite regularly before mounting up to help calm my nerves. I'd take him into the school tacked up and id wander aimlessly, changing direction on a whim, stopping, backing up, moving him over and I wouldn't get on until I was happy we were in sync and he was listening.
Spook busting is also a fab exercise, there are heaps of things you can introduce your horse to, I did such stupid stuff, wandered round my field covered In a tarp, put balloons up, umbrellas, my gym ball, plastic bags. It's all fun stuff and could be done day to day rather than as a proper schooling session.
It will help you see your horse's reaction to novel objects and along with walking out inhand it will help reassure you of what reactions you can expect.
Mine had a run first think later mentality and I eventually started clicker training to get my boy to investigate stuff. They put new farm machinery in the corner of a stubble field near us and I took him up to it and did clicker to get him to touch it. Now any time I bring something new near him, he's hopeful he's going to get feed and is happy to nose it. But then he now trusts that I'm saying it's ok as well so he's more confident in himself. Shove someone else who doesn't give him confidence though, and he instantly returns to his former self. Half of it is him being a chancer as he wants to go home, the other half is genuine worry. Horses are such wonderful creatures and it sounds like yours is going to tell you exactly when you're doing the right or wrong thing. If you're not getting the response you want, chances are you're not asking the right question and need to look at your own actions. That will make it so much more rewarding when you do get it right!
Good luck, sorry for the essay and keep us posted x