Do you feel very tense while in the saddle? I bet you are bracing and don't even realize, as prior to your accident, you probably sat the trot without even a thought due to no stress in your body/mind...after an awful fall, with a long and no doubt painful recovery, I bet you have a lot of fear (I WOULD!)
So you can't expect it to feel like it used to...yet!...But no doubt your mom has also explained, it WILL COME BACK. Try not to focus SO HARD and just relax and "go" with it. After not riding for 15 YEARS, (my story) it took me about six to eight hours in the saddle and practicing ONLY WALK AND TROT, alternating between rising and sitting trot...sitting was the hardest, though controlling the horse's speed at the gait was one of the best methods for dealing with it, keeping her at a nice, slow, easy trot until I would establish a rhythm, and then I kept thinking, "breathe, stay loose, breathe, stay loose", etc...
Another thing that might help you as you are surely coping with a lot of residual fear, is maybe having your mom (she gives you your lessons, right?) put your horse on a lunge line and ride in a round pen of you have access to one, and that easy, you can just focus on your body, and relaxing into your horse's movement, and not worry even about having to steer, etc...in my case my trainer had me close my eyes while on the lunge to refocus on the rhythm of the horse, but you should get it much faster and with less effort than I did as you've only been out of riding a very short time (& I REALLY THINK you are likely bracing due to fear and preparing to fall again, even though you are very unlikely to do so!). If you are tightly bracing all of your muscles, especially your inured hip & your back, you will bounce a TON while during the trot.
I wager a bet that over your next few rides, you will have this handled! Remember that you are fine and are not likely to fall/be thrown again, and that you love to ride, that you are safe, and just TRY and move with your horse!
I wish you so much luck! What you are going through is SO NORMAL, just remember that, and that you CAN sit the trot, you've done it all your life, and soon you will do it again! :0) ~ B2H :0)
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