Would it be possible to start Higher Level economics not and get a good result in it?
Of course. You shouldn't doubt yourself or your capabilities if you really put your mind to work.
Get learning now. Start with microeconomics and then move on to macroeconomics (trust me this order is best).
Build on certain key topics and really try and understand them. Use your past exam papers and do questions based on the topic you just learned. Don't try and do an entire paper now, take it topic by topic.
Here are some useful resources to help you:
This is a youtube channel called ACDC Leadership and the guy on there does some great breakdowns of economic topics. His videos are all fairly short, entertaining and to the point.
Also, try viewing some videos from Khan Academy on economics if you ever need more information on a topic.
All the best.