Try adopting a light seat to get you going, that way the horse can just come up into you and you'll get the feel back. If you're worrying about being left behind then probably tensing or over compensating coming into the fence which will only make matters worse.
Once you get your eye in again and are a bit more confident, sitting down and riding for the fence should be a lot easier x
If you are getting left behind, it's most probably because you anticipate the jump and try to be in front, strangely enough.
So, a two point seat, look up and far past the jump, wait for the jump to come to you and let the horse 'take' you over it.
Also, when riding in two point, it can help to grab a piece of mane slightly farther up the neck which will in turn save your horse's mouth if you do get left behind. Let your hip angle close naturally with the horse, and you'll soon get the feel.