Hey Sopie Sarazin,
The reason my position isn't that good anymore is that I did Saddleseat for 5 years and while doing that I did some huntseat and jumping. I never really got an actual jumpi g lesson until last year when I had to stop saddleseat and go to jumping. In one of the lessons I fell off of the horse because he bucked right when we landed and continued so I broke my ankle and couldn't ride for 3 months. I've been doing huntseat since then but just got back into jumping. I'll be sure to use your advice.
Keeping your horse in front of your leg will make it easier, then when you take off just make an extra effort to look up, this will automatically bring change your balance, making it easier to keep your heels down also just thinking about putting your heels down when you take off helps :)
try trotting without stirrups. it will strengthen your legs so your legs will stay steady
Don't grip with your knees and put the ball of your foot on the strup pad. Also stand up in your saddle while squeezing with your calf and sit down with out moving your leg. Then try 2-pointing without moving your leg and put you weight in your heels. Hope this helped!
I would suggest practicing without stirrups, that gets your legs stronger and also think about keeping some of your weight in you heel and not in your toe. Do a lot of flat work also!
If you had a good position until recently, I don't see why it would change ;)
Though, I would suggest you to train with poles first so it's easier to focus on your position and on keeping you heels down. Don't forget to relax you knees so they don't squeeze.
Always keep your look high so you body don't go forward.
Good luck :)