She is a baby. I would be amazed if a springer this age was anything but a pain in the arse 
. The bad news is, she is likely to be so for several months yet. Your springer is not going to be properly mature until about 2 years old.
Battendog is a springer who is 15 months old. He:
- walks ok on the lead in environments that are not overly exciting. As soon as he sees another dog, gets another smell or spots something he wants to investigate his first reaction is to try and drag me over to it.
- didn't stop biting until about 9 months old.
- wasn't reliable at toilet training until about 5 or 6 months
Springers take while to mature!
I would get her on a harness to stop her hurting her neck. Use the harness any time you are not actively training her to walk nicely.
I'd also find a decent trainer. Honestly, anyone worth their salt would not have allowed you to spend hundreds without giving you a way to work on this.
I personally use a combination of stopping dead anytime battendog pulls on the lead (think of the lead like a brake, he pulls, it stops me). I only start moving again once the dog has slacked the lead, and they will do so eventually - even if it takes 10 mins of you stood like a statue. This is why you need to be clear when you are actively training, because the first hundred walks are going to be slooooow :).
In the early days I would also use a treat to lure him back to where I wanted him to be (by my side) and still use treats to reward him when he is there.
We have been doing this twice a day, every day for six months and I expect to need to do it for at least another year, with small improvements as we go. Loose lead waking (for some dogs) is a long haul.
I also found that Battendog learned much better when the lesson was fun and exciting. For loose lead walking this meant that when we are walking along I try to make being with me as interesting as possible. We stop to sniff things, I change direction often, I sometimes speed up a bit, I often reward and I praise liberally.
If I took him to classes where he just had to repeat the same exercise over and over he lost focus and was a pain. I took him to agility where the work is exciting, fast paced and different all the time and he shone like a genius.
Work WITH the dog. Think about all the things she is designed to do and understand that she cannot resist doing them. She is designed to run and flush and sniff. She is designed to be absolutely focussed on those activities and to never give up until she has the bird. She is designed to bond closely with one person and want to roam the fields with them. Think about ways you can do versions of those things with her.
She will need plenty of mental (and limited physical) stimulation. At her age Battendog was almost a full time job! I would expect a springer that age to need 2 or 3 short walks a day, along with about the same number of training sessions, lasting about 10 mins plus one or two good play sessions. Something like that.