How long have you been around horses just this year a few months if so your not ready to care for a horse so maybe go one step slower lease! If you haven't already its a good way to show your mom that you can care for a horse by haft owning a horseonce you show your mom you can take care of a horse by leasing she may change her mind!! Even if you have been riding a long time if you have try leasing first she may change her mind!
did you include... farrier visits every 5-6 weeks vet bills if your horse were to collic medical cremes and gels for your horse if he gets a scratch or something like a girth sore monthly cost of board halters/lead ropes riding tack
you'll need all this and more, your parents know whats best for you...just take their lead
Maybe you could tell your mom that you want to adopt a horse -and actually adopt a horse- to help animals. Instead of buying from a person that has enough money to pay for the horse temporarily adopt a horse and help that agency and you community.