Red-1
I used to be decisive, now I'm not so sure...
I would not continue with the share but I would try to leave on good terms. So it would be more, "Thank you for allowing me to share your horse, I have really enjoyed it and realise I want to do more now. Because XXXX can't do more, I think it would be best if I give you one week's notice now, and look for another horse. Thank you for helping me get to that point."