You didn't advise your dogs age, it's training level, and what, if any, program you used. These are all critical to giving you more than a general answer.
Steadiness goes directly back to poor or non-existent Basic OB. I'm going to take a guess that you didn't train your dog following a program. Failing to fully train & follow a program sooner or later bites a guy in the butt, when the dog decides to do what he wants and you have no foundation or tools to correct it.
I also might hazard a guess your dog is young, maybe too young to actually be hunting yet. As a rule it takes 16-24 months to take a 7 week old pup through most programs, so realistically their first actual hunting season is when they are around 18 months old.
Basic OB is the foundation of all training. This is where the dog is taught that you are in control of his actions and to stay where he's put until released or sent on a mark. In effect, your dog has learned that whether or not he sits still and when he takes off on a retieve is his decision, not your's. It sounds like you let him do this, so his failing is directly your fault as you have reinforced it by giving him tactic permission to break.
I'm going to guess that from your description he hasn't been taught to Handle, either. As such, you have no method to communicate with and direct him when he's away from your side. Once he's away from your side, he can just do what he wants, i.e. look for something/anything to retrieve because it's fun, not a task to be done for you as you direct.
Hopefully you have actually Force Fetched/Collar Conditioned your dog, and didn't just put an ecollar on him, so that you have a tool in place to correct him that he understands if he's not at your side. If not you are at a training disadvantage and virtually helpless to control him in the field once he leaves your side.
You need to go back to Basic OB. If you haven't formally trained OB, it will take about 4 weeks to put in place. If you have, then you need a couple weeks of OB reinforcement, until he's rock solid.
Then Basic Marks where the dog doesn't move until sent on the retrieve. Use the ecollar if he breaks, put him back in place, then go get the bumper yourself. If you haven't actually CC'ed your dog, you can stake him with a 4' lead. He'll hit the end if he breaks, you then correct him, put him back at heel, then again YOU go and pick up the mark.
Once you have him steady at this level, re-introduce the shooting the shotgun prior to being sent. As he's using the shot as his signal to take off, not your command, expect him to back slide & break. Keeping working as described above.
Nothing a gundog hates worse than not being allowed to make a retrieve
and nothing reinforces steadiness better. Also, the dog has to know that every mark isn't his by default.
I think it's fairly safe to say you probably have the better part of 8 weeks of twice a day training ahead of you to fix this. NO hunting for him until you do. Don't hunt him again until he's steady, then for a few times have someone else do the shooting; your job will be only to work with your dog.
Personally, I hate dogs that break & have poor OB and refuse to hunt with them, hurt feelings be damned...