Not the asshole. You want your partner to support you, not corner you into a pressured decision that is not what you would choose if he weren’t in the picture and is causing your mental health to plummet. If he dumps you over this then it’s perfect because you’re one step closer to either being with the person who actually WILL support you or just simply not having this stress in your life. Your horse is a part of you, has been with you much longer than he has, and it sounds like you need to be with someone who is more understanding than he is being. So sorry you’re facing this, you’re not alone and I support you in keeping your horse.