I dont think you should buy a horse if you keep changing your mind, its not fair on the animal. You have only been riding for three years and therefore have little experiance in horsecare. I was riding for 10 years before I got my horse (4 months ago) and im learning every day even though I thought I had prepared everything before he came home.
Keep taking lessons (like Speed Racer said), then if you get bored again then there is no real harm done. Try doing client shows/ competitions of your stables do them and see how you feel competing, if you want to go further then lease a horse first, keep the horse at a barn on livery then you will have a little help with the care side of things and there will be plenty of people to give you advice. Good Luck x
Keep your feet on the ground when your head's in the clouds.