This is good as are the other suggestions. I have been there as have most. You will not correct this problem if you dont get into a training group and replicate a test. I train with a group on average one time a week in the spring and summer. I always run my dog last so he is wound up tighter then a clock when its his turn. SLOWLY walk up to the blind with a tight heel. On average for every 3 steps forward I will take one or two steps back. I never want him to assume we are only going forward. If you give him an inch in any of this you can bet your next paycheck he will take a BIG mile. Stop running all trials and tests until this is fixed or it may get to the point where you can no longer fix it. Look at your long term goals which will make not running now much easier.A very common problem especially with new handlers. Not laughing at you but as a former judge, I used to smile inwardly at these holding blind battles. To me the TEST STARTS IMMEDIATELY when you "invite" the dog out of its crate or dog box. Break the test down into sections. Dog is now out of crate and must SIT. The lead is now on and the dog MUST HEEL and not pull you to the HB. I face the dog looking out from the blind, especially the young dogs. While they should be sitting many want to look under, around and even jump up on the HB. When called to the line remove the lead and tell the dog to SIT as you walk out of the blind and away a few feet. Again, "invite" the dog to now join you by saying HEEL. Keeping a high OB standard is easier to maintain if you start with the dog coming out to the dog going back into the kennel.
Condition your companion on each of these sections until their perfect - then rinse and repeat as needed throughout the dogs life - maintain that standard.