I step on my little dog without wanting to, and now he has no courage, he is breathing very badly. What can I do to improve it?
It's possible that he has some internal injuries you couldn't notice. You should take him to the vet as soon as possible to examine him and determine if everything is fine.
Normally, sudden changes in dog's behavior are usually associated with a medical problem and feeling pain. The first step should be to rule out any medical causes before considering it could be a behavior problem (prolonged stress and anxiety, fear, reaction to external stimulations, etc).
You should contact your vet to rule out medical problems as soon as possible.
All the best