I have a similar situation and it is entirely possible you are more capable of figuring this out, OP, than you might think, because I’ve done it. I’m no superwoman, nor am I all that talented in the saddle, I’m just extremely determined! My trainer is an eventer, so dressage is our flatwork- this is 100% the most important factor because without learning how to use my seat as an aid along with all of my other aids, all the leg in the world wouldn’t get my horses in balance.
I’m 54, I’m pretty little, I do the jumpers but spent my younger years in the hunter ring so I came with that forward seat thing installed, I have two horses, AND I had to learn to ride like a German about a decade ago when I got my now-older guy. He’s built like a big tube and he’s lazy, so getting him to work over his back is hard for me because he doesn’t want to use his core. He also has a very, very, very powerful hind end, which is great except that he can pop me up out of the saddle and dump onto his forehand at the canter unless I have him really engaged and sitting and light in front, and I’m really engaged and sitting, lol.
When this doesn’t happen, he’s heavy in front and his hind end is out behind him, so we are unbalanced and it sucks. I can jump him around 1.0m and even most 1.10m jumps all day long like this because he is scopy and powerful, but anything higher or more difficult (a wide 1.10m oxer off a short turn, for example), and it gets harder physically for him to jump unbalanced so he…well, as he’s gotten older he just won’t jump.
To develop more strength in myself, what I did first was started running 4-5 times a week, 20-30 minutes, on a treadmill that had an incline function. When my knees and back started protesting that (I also have scoliosis), I got an air bike and now I do sprint-slow pedal-sprint-etc. on that for 20 minutes, 3-5 times a week. Nothing replaces time in the saddle, but this has helped my general fitness immensely. I was also lucky enough to be able to get another horse two years ago, which gets me in the tack more and in the show ring more. So, now I: have two jumpers that I ride 5 days a week, 6 days in the summer, ride my bike fairly consistently, and feel like I’m riding better than I ever have before.
The leg thing is so important, but with this everyday focus on dressage I’ve also been able to understand how to get my big guy lighter in front and that is the other part of it. Leg into hand, he goes from a downhill rectangle to an uphill square, and so long as I have the right forward pace, we can jump anything from any distance. He actually prefers to jump from the base, which of course needs him to be VERY engaged over his back and sitting. I do that and he doesn’t care where we take off from because he has the power and pace and balance to get over whatever and it feels very easy to him.
I will also add that this one’s default is always, every day, to be heavy in front and not working over his back. So, every day, I get on and start our lateral warmup work (leg yields, shoulder in, shoulder out, haunches in, walking turn on the haunches, regular turn on the haunches, half pass, etc.) and when he ignores my leg, which he ALWAYS does, I tap him with my dressage whip and that sharpens him up. If it doesn’t he gets a smack and then a quick pat as a reward for going forward, and then it’s back to work. Other things that help are to trot, ask him almost to walk, but then add leg to go back to a working trot, several times in a row. In the canter, do a medium canter, then collected, then medium, down to a trot, back up to canter, etc. Once he knows I’m serious about him actually listening to my leg he is 100% lighter in front and everything gets much easier for me, physically.
So, training and then reminding him to be quicker off my leg from the moment I get on has really, really, really helped because now I’m not just endlessly kicking into the void of his “la la la la I’m lazy I can’t hear you, lady” non-response! My trainer can get on and immediately put him together because she’s a better rider than me, and is stronger and younger. Getting him to listen to me involved figuring out that he was ignoring my leg primarily because it just isn’t as strong as hers, and that helped us both understand how to fix it.
My little guy is, again, much easier to put together but he is also built like a tube and prefers to go on his forehand, so I ride him like a German, too. It’s a less physical ride than my older one, but it’s the same basic setup, and without having a strong core myself I honestly do not know if I could ride either of them successfully to bigger jumps. They have very powerful jumps and you need some strength to jump a course with either of them! I have long legs but a very short torso and I am by no means an Amazon- I’m actually sort of scrawny.
Anyway, sorry for the book, this just resonates with me because I am in the same boat to a certain extent. If I can do it, OP, there’s a good chance you can, too.