I did my Higher English last year and found the prelim difficult and had very low marks. Most of my class didn't appreciate the step up from NAT5 to Higher and many failed the prelim as they were just not ready and didn't have the skills for a Higher English exam yet. Looking back NAT5 was ridiculously easy in comparison, Higher asks a lot more of you in all of the different components. By the time of the final exam, if most of us had gone back and sat the same prelim we would have scored much higher. It wasn't the prelim that was too hard, we didn't have the skills yet and in all honestly had not prepared enough.
Don't worry too much, take it as warning you need to understand what is required at Higher and you need to study and practice the skills needed. I was terrible at English so spent more time studying it than any of my other subjects, it isn't easy unless you read a lot of challenging texts normally or you are just a natural at it. The marking instructions on the SQA site for past papers is a good place to start for RUAE and Scottish Text which is a similar skill. Also work through some of the revision guides (How to pass, Brightred, Leckie etc) as they show you how to break down text, answer RUAE questions fully and get those extra marks. Start reading challenging texts from quality newspapers or magazines now on different subjects, at least one every other day, so you get used to them and don't panic when you get a tough one, it does make a difference.