I'd suggest trying to get a few private lessons where you and the instructor can concentrate on correcting your position, not just telling you what's wrong.
In an ideal world you should have a straight line from ear - shoulder - hip - ankle and the front of your trunk should be the same length as the back. As you can see you're missing nearly every point. You have a chair seat, ie legs way in front of you, and that has pushed you to the back of the saddle. You're leaning forward probably because you feel insecure with where your legs and seat are, but you've also collapsed your middle which further weakens your position. Imo security comes from the leg and seat, and I bet if you could "just" sort your leg position out a lot of the rest would fall into place because you'd immediately feel more balanced and with the horse as opposed to bracing against it.
If your RI can't or won't help you with this - and saying legs back/heels down/sit up is no help whatsoever since you don't know how to do that and if you did it wouldn't be an issue - then I'd suggest going somewhere else for some private lessons. Explain the problem before you go, and be prepared to spend time in halt and walk getting it right, it will make a huge difference to all your riding.
I suspect I've sounded like I'm tearing you to pieces and that isn't my aim. It can be fixed! And the first step has been taken, namely realising it needs to be fixed. Go for it!