He looks to be a wonderful learning mount for you Carly.
I HATE the word "never" but in this case, I agree with Hayley/Rocklone. As a youth your age, I sometimes didn't have a choice BUT more often then not I simply was too lazy to put horses up or to go thru series of gates to get to "empty" riding area(s). Sometimes those choices led to injuries, torn clothing, damaged equipment - more often my own due to being hit by teeth, front or back hooves when the alpha horse or a playful horse came charging up or ran alongside, bucking and kicking. Two times that I recall, my mount ended up with injuries and in one case they were serious - vet bills and months of healing/daily care and then later bringing the "young" green mind back as that horse was terrified of other horses for even longer than the months that I had to care for her.
That said, I do to this day, sometimes ground drive, drive or ride (last time I rode was in 2012) in a pasture with loose ponies/horses. But, I'd like to think that I've got more experience these days and can judge who is loose vs who I'm working so as to benefit everyone without getting any injuries or even mental stress/downtime. In fact, I choose to work some horses in a specific pasture or paddock to either get the horses in the pasture experience seeing a horse being worked (and at more than a walk) OR to give the horse I'm working a chance to accept loose "objects" in their work space... I also will spend time working a horse past/thru a barn so that they understand just because it's there doesn't mean we are done or they can stop OR past the hay feeders for the same reason. Same thinking, if I find an area they are constantly shying away from or getting totally nervous I'll take they a ways past that object, work pretty hard - until they are warm/sweaty and huffing a bit, then back to the spot that made then nervous to stand and get a rest/breather.
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It's wonderful to have a horse to "toodle around" on. And a great way to end a working session and to help with developing balance.
However, you appear to be very tense - locked up in the lower back, hip/thigh area and it's carrying all the way up to your shoulders, neck/head and out thru your hands...
Consider staying on the lounge for a while (interspersed with other riding when you don't have another person to help you). Practice riding - starting at a walk. No reins in your hands - either remove them from bridle, wrap then into his throat latch so that they stay up by his ears or knot them so that they don't flop around and make you want to grab them.
with dropped stirrups crossed in front of the saddle or removed -
1 - Legs Away - grab the front of the saddle (pommel/gullet) with both hands, tip back & pull your crotch towards the pommel. start w/ your knees fairly close in (if you can?) or even resting on the saddle below your hands and just have your lower legs away from your mount. The goal is to eventually be able to raise your knees and lower legs away from the horse - literally opening up your pelvis and your seat bones, strengthening your lower back and ligaments thru the hips & pelvis and teaching your mind that you can balance. Myself, my students and my children all worked at this long enough to be able to go several rounds on a 25-30' line at a trot & canter (on some of the mounts that the girls learned on, babies, it took much longer). Works best with a "steady-eddie" type horse with smooth gaits, but also sounds like Hot Shot is. (and no, I don't recommend having lots of folks watching this one as you learn it. The wise cracks can get stupid and ridiculous and are so TOTALLY UNNECESSARY and undeserved. This is a favorite exercise for me to teach to all ages - it does a LOT in a very short amount of time - all at once. Believe it or not, because it is forcing you to tip back, it then develops your balance so that you DON"T tip back while riding or if you get "behind the motion" at any gait, your body is strong enough to bring you back up to center - also a form of muscle memory)
2 - while your mount is walking (& later trotting and cantering) - sit up straight and "walk" w/ your legs as well. Again, this will loosen and strengthen the ligaments in lower back, hips & pelvis and also does amazing things for your balance and confidence.
3 - neck stretches - tip your head slowly down/forward. Then tip as far back as you can - you may need to have a pommel strap to hold on to to get the full stretch safely as you are able to. Then tip first one way (side), then the other. You don't want to raise your shoulder to your ear - but drop your shoulder down and literally tip your head to follow it down, letting the weight of your head "pull" it down... Do not do these stretches at a trot - can cause the opposite of what you want - tension and possible injury. I have at times down them at a canter on horses that have nice, cadenced, soft gait. Finish by rolling your head down/forward and on around to the opposite side - working on smooth steady movement w/o jerking. This one actually goes a long way to helping to relax in a tense situation.
4 - Remember those pesky and sometimes painful arm circles? Do those now in sweeping movements while your legs stay relaxed (at first your legs/back won't be relaxed - the goal is to get there!). Your eyes should be up and forward - your head should be turned slightly in the direction your mount is moving. When you start, if out of balance/fearful/feel like you're falling, hold pommel with outside hand and do only one arm. Do the circles big and sweeping - 1st forwards then back. Then make the circles smaller and hold the same size for a couple then smaller still. Switch hands. Take a break (do something else), then again switch directions and do it all again. When you become good at it and feel you are comfortable with your balance - circle one arm forward while the other circles backwards (harder than anyone thinks, LOL).
5 - raise your arms to shoulder level and turn your torso with your arms staying even. Turn so that your body is facing inside the circle-towards the person on the ground. Then steadily change and move back to center. Then move to face the outside of the circle. When you start, you probably won't get very far. Keep your movement steady w/o jerks (children for some reason that I haven't figures out, do these moves in jerks at first. Some never really get past that until much older, think it has something to do with body/motor control). No worries, keep working on it. These can be done at the stand still, walk & canter at first - working up to doing them at the trot (much more difficult - especially if posting - but can and should be done eventually).
These exercises sound so simple but they DO loosen & relax various areas while also strengthening. They develop a deeper, stronger seat and independent balance which in turn boosts self - confidence. As you get good at them, you will find your posture improves and so does your walking on the ground. If you weren't previously good at dance or gymnastics, you may improve now. They do take some time to see/feel all the improvements but they are there almost from the first day. Don't push so hard that you become extremely sore - that defeats the purpose, takes longer to work thru, actually has the opposite affect on confidence and I don't feel that is the right way to train your body. Also, they will not necessarily cause weight loss but they can cause some changes in size/shape as muscles become stronger OR they may not. It does vary from person to person. The main purpose is developing flexibility, strength & confidence while losing the tension that can cause your mount worry and you to get injured.
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I also teach standing in the stirrups at all the gaits - and not just for learning "two-point" - a position for long gallops and jumping. It will build balance & strength in necessary riding muscles. Later, it's a great way to both give your horse and you a "rest" if out on a long or very strenuous ride. I utilize a neck strap - about 1/2 way between the withers and ears - to hold onto for support or to grab if you lose your balance rather than the pommel or horn as that puts your hands too low and "under" you - causing you to fall backwards, imho. You should learn to hold the neck strap with a hand on each side of the neck and also independently with either hand centered over the neck (learning to lengthen your arm, not twist your body) AND to balance with no hands on the neck strap at all (with arms relaxed down & hands on your thighs; out to the side at shoulder height or crossed over your chest).