What has been said about core strength. Time for the gym, and some coaching on what to work on at home. It will make a big difference.
Also, watch William Fox-Pitt videos, just one of which was linked above. Notice how, in jumping position, just how far his butt is behind the cantle. WFP rode many horses of different sizes and temperament (a rider who REALLY rides, as some riders just ride different versions of the same type of horse). Keep searching the videos and you can see him many times, dressage, show jumping, everything, on various sizes and proportions of horses.
And, find an instructor who is your shape. With all due respect, a good instructor can help riders of different body types. But only someone who is built at least somewhat like you really understands what you are feeling.
I have various body parts that are proportioned differently than most people. It’s not something that’s obvious, except to the tailor to whom I turned in desperation to get clothes that actually fit. I found that an instructor whose body type is significantly different than mine just isn’t that much help.
As was said above, find the right role models and you can find what will work for you. 
And just for what it is worth … although there is a general body type that is often found among 5* eventers (between 5’7"-5’10", 140-175 lbs, very fit and strong in the core), the 5* masters come in a wide array of body proportions. From WFP to Jonelle Price to Ingrid Klimke to Oliver Townend. That’s true up and down the eventing levels. There is something for everyone to watch and learn. 