First of all, I LOVE your attitude! You are willing and able to listen to the help you are being offered and stay positive about that. Do you know how rare that is?
The advice you have been given is very good. You need to relax your whole arm/shoulder so that your hand can unfreeze. You need to be able yo absorb all of the movement of the horse's head with your arms so that you can maintain the SAME amount of contact at all times. Contact will change while giving a half halt, but at all other times there must be 100% consistency in the amount of contact you keep.
I suspect, from the photo, that you lock your arms, which will lock your hand. Then, when the horse moves its head, it will hit that bit with every step. That is very destructive to the acceptance of the bit. The horse will be uncomfortable and will never want to move onto the bit and relax.
Here is an exercise I will do with a new student who has this problem. You can substitute any friend willing to do this.
I will stand at the horse's head facing the student.... I will hold the reins in each hand near the bit....I will have the student hold the reins as if riding.....I will tell the student to apply contact......then I will move the bit forward and backwards slightly, mimicking the movement of the horse's head while walking.....then I will have the student unlock their shoulders and arms until they can follow that movement while keeping the SAME amount of contact at all times.....I will keep moving the reins until the rider can absorb all the movement while maintaining that contact......then I will have the rider practise changing the amount of contact while still absorbing all of the movement.....keep doing this until the student can adjust contact immediately and keep consistent contact at all times.
This will really help, but locked arms are a difficult habit to break. It will take time for it to become a new habit. Keep at it, though, because the horse can never fully accept the bit until they have a kinder place that they can trust. The amount of contact is rarely a problem for them, as long as that contact is consistent.
As for the long and low, it ia a way of asking the horse to stretch forward into the bridle with little contact. The horse is relaxed, moving nicely forward and seeking the bit.
When working on the horse accepting the bit, if the horse is flexing the correct muscles going onto the bit, these muscles get tired quickly, at first. It is good to allow them to stretch often. I will often do this exercise with a green horse. I will ask the horse to move forward onto the bit, asking them to flex the correct muscles. I will keep them moving nicely forward to exercise these muscles. After a moment, I will slowly allow the reins to lengthen, still moving them forward. If this is done properly, they horse will maintain his balance and be able to move longer and lower. If they lose their balance, they were not properly forward and will fall onto their forehand. I keep working with them until they can stretch longer and lower and still maintain their balance.
The horse will be moving much like this,
As you can see, he is stretching forward still moving into the bit. He is comfortable with the contact, as it is so consistent it never bumps him in the mouth. He is seeking to maintain that contact. If he hot the bit, he would back away from the bit and cease moving forward.
BTW, If I were to critique my own photo (which I often do to illustrate a point as we can't always be perfect) I would say my hands are a tad low. I should have them an inch or two higher to have that straight line from elbow to bit that I always want to see.
To show how important this contact is, I had a student at a clinic (she allows me to use this photo) who just couldn't get her hands moving well with the horse. As a result, the horse wouldn't move forward into the bridle. The horse has zero impulsion moving forward.
I got on the horse and within about five minutes the horse trusted the bit again.
After doing the exercise I explained above with the reins, I had the owner better able to have more consistent contact.
Her hands are in a much better position (still a bit too low, but this is, as said, a hard habit to break) and are starting to move with him. The horse is beginning to trust her more. After the trust is there, then she will start asking for more impulsion, which is missing here.
I hope this helps!