I think about it like no one is perfect horses can have a mind of their own so if you mess up just learn from your mistakes and use it as a learning experience!
just tell yourself that it is only for practise. nothing is on the line you don't HAVE to make a good round. and i would just start over. start with a confident horse over small jumps so you get your confidence back up.
When I was a junior I would catch ride sales horses but this ruined my confidence because a lot of them weren’t well behaved and would stop and act out in the show ring.