What a tough situation for you to be in. Even experienced handlers are taken by surprise.
First off, be thankful that nothing serious happened.
My thoughts based on what you wrote:
If you know that Sib will "fence fight" don't give him full leash - when you are walking near a fence (regardless of whether he will challenge it or not), reel him in close to you. This way you have better control over him. It's much easier to control a dog on a 3-foot leash than a 10-foot line.
If you can, talk to someone that you know has a dog "behind" their fence. See if they can let the dog out while you and Sib are walking by - short leash. Stand there and watch your dog's reaction - and be ready to correct / interrupt BEFORE he gets revved up. Make him sit / make him focus, whatever it is you do to work with him to get his attention off the fence.
I worked on this with Kyleigh when she was very young. I would literally stand on the sideway while the other dog was barking up a storm (either behind a fence, or from the front window), and I would play with her, engage her, whatever until I was the MOST interesting thing in the world. After a couple of sessions - she will walk by ANY yard regardless of what's going on - she'll look over, but there is no more "reaction."
Yeah, you screwed up ... so what? We all do, we all will continue to ... you made a mistake, move on ... trust me, your dog is not holding a grudge because you screwed up! Only you can make yourself feel worse.
Your dog will only be what you want him to be ... you have to have the expectations of your dog, and then the willingness, patience, consistency and knowledge to follow through on everything.
Have you thought of getting a professional trainer for some one-on-one sessions - even one or two sessions can be VERY helpful?
Marion’s Zoo-Kyleigh, Raylan-cat, Echo-TAG,