Re: argh! I'm so bad !!
Is there anyone who can film you or take photos of you riding? I used to have lessons and be told that I was leaning forwards. My instructor would say, "Sit back!" and I would swear blind I was, until I saw myself riding. I agree with Cocosmum, because at first when you are told to sit up you will feel like you're tilted backwards, but in fact you won't be, you'll be upright. If you can see photos of yourself, you'll see the difference, then you'll get a feel for the correct position and you'll be able to correct yourself.
Unfortunately, when we're nervous, our instinct is to curl up into a ball for safety, so if you're nervous riding, you'll tilt forwards. If your stirrups are too long, you'll tilt forwards. Pretend you're in a scary situation while riding in the school and tilt forwards, take a look at yourself, because everything about you will be wrong. Then SIT BACK, take a deep breath, put your legs down straight, breath out calmly and smile, and say to yourself, "This is where I'm going to be! This is my control point!" and feel confident and ride on. You'll soon get into the habit of being confident and sitting back, but it helps to make yourself do it, first.