Thanks for your replies!
Firstly NittanyEquestrian- it's a bit of both, I sometimes think she wants a itch on her leg or going to cough so I let her head down and she goes for the grass I try and pull it back which makes her rear. If she's not allowed it from the start she is normally ok its just I need to be quick enough to stop her getting the grass.
Northern Mama- its mostly when she has her bridle on as that's when I don't want her eating. I don't think its poor fitting bit as she has had it for a long time and only rears up when she can't have grass not when I just pull on the bit. She is my first horse of my own but I've been riding for 17 years now since I was 5. I've had ella for 10 years now and I think we understand each other but maybe a bit of join up and reconnection might help things.
Jewls17- i'm sorry but im glad your not my pro trainer- you say you find it easy to stay on a rearing horse but it really ain't! I'd rather get off the horse volunterarly than be crushed underneath her when she topples over backwards!!!! And hitting a horse is really not the answer either as I have tried this before and as soon as she hits the ground she is back up again! And it definitely has nothing to do with her being fed enough she is very well looked after and gets plenty of feed!
I have now read up on it and most people say to keep the horse off balanace as they can't rear up when they are off balance so I will try this next time I ride. I'm not staying off her as she is a sweetheart really and its so rare that she does it, I will take in everything I've read and heard and hopefully next ride will be successful