When I was 8 (I'm 15 now) I convinced my parents to get back into horses (all of my family has worked around horses for most of their life) so, of course I started taking lessons, we found an older horse for me to own (she was 18 at the time) and everything was going great....until at the end of the first week into my lessons they started having me bareback ride on a 4 year old gelding. He was good horse, don't get me wrong but he was just a bit to young for me so early in the lessons. So I started getting more and more nervous and less confident over the next week (i had lessons everyday for 3 hours it was more like a camp) and by the end of the second week, during the last "camp style lesson" he bucked four times. I stayed on but it completely shattered all my confidents. Of course though, since I had just gotten my own new horse my parents expected me to ride her so for the first month my mom led me around on a lead rope until I got more and more confident in my self and more horse. So basically, just take it slow. Don't try to live up to anybodies expectations but your own. Start out grooming, make sure your comfortable with that before anything else and build a relationship with your horse. For your first ride have someone lead you around while you just get used to everything, if that's what you need, then build up to you riding the horse alone in a small corral, their no hurry but try to push yourself little by little

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What really helped me though is my horse, she was an 18 year old quarter horse mare (still have her, She's 25 and still going strong) who was more of a 1 person horse, and after we built that relationship it made a world of difference. Having a horse follow you around, having a horse that trusts you is just amazing and boosts your confidence so much. Don't worry so much about age or breed or gender (just don't get a stallion), just look for a horse that kid broke, and been through everything. Make sure their also broke for more advanced riders so when you do get more confident they'll push you till your able to ride around in a pasture, full gallop completely bareback with just a piece of twine around their neck as reins (its so much fun :P)