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It sounds like the horse was trying to evade your cues.
get into the habit of squeezing and releasing before the horse has done what you ask.
This was your first time riding this horse, so it was ok this time. You weren't sure of his limits, so you backed off. I have a feeling that you were squeezing/releasing while you were trying to get him to go. I had this habit when I was taking lessons, and I turned my lesson horse into a nightmare to get going. I would squeeze and kick her to lope then I would stop when my instructor would talk then try again. I was unintentionally telling the horse, "I want you to go faster. Ok, you don't have to. Yes you do. Not now. Go. Nope. Go. Nope." My horse, in turn, was saying, "Well, since she can't make up her mind, I'm not going to go any faster. She'll give up eventually." It took an entire lesson to "fix" her.
Bottom line: Make sure you are keeping consistant pressure on your horse.
Now to deal with the backing:
If your horse is backing to avoid work;
- Make sure all pressure is off of him, then say, "Woah".
- If they continue for 3 or 4 steps, make them back. It's like saying, "You want to back? So lets back!!!" (Make sure you tell the horse in a manner that says you mean business)
- Back a good 5-7 steps, then spin your horse in a quick, small circle. (bring the horses head around to you and disengage his hindquarters.)
This should get rid of the problem, because eventually the horse will realize that backing to aviod
his work is only creating more work for him. If the horse continues to back, keep repeating the steps until your horse is focused on what you are telling him to do.
Hope this helps!