Accidents happen so it's not your fault! It's great someone else will still be exercizing your horse for you!
You can spend all these months to make a tighter bond with your horse. I'd suggest to work with him in freedom (like lunging free in round pen), teaching him to obey voice command (whoa, walk, trot), etc.
"Life isn't about waiting for the storm to pass: it's about learning to dance in the rain..."
"When we are no longer able to change a situation - we are challenged to change ourselves."
"How people treat you is their karma; how you react is yours."