Thanks for all the thoughtful and kind replies. I'm crying, which is a bit embarrassing as I'm in the cafe at a railway station..!
To respond to some of the points made:
i asked myself if this was sibling rivalry, and, yes, that will be playing a part, though I think I'd have still felt pretty vulnerable if it had been a friend. She's my younger sister, and the older/younger dynamic was the reverse of that in many families. She's very talented, attractive - and like my mother, who I think took one look and fell in love! I was always second-best after that, and have had to deal with the consequences over the years. No sense of superiority coming from my sister herself - she didn't make the suggestion about the quilt to show off, just because she's creative and saw how to make it better. She's done a bit of quilting and a wide range of textile and paper-based work over the years, always original rather than patterns.
i was pretty useless at anything to do with arts and crafts as a child and until I was 50 firmly believed that there was nothing practical or artistic that I could do. Then I tried and, to my surprise, enjoyed, crosstitch, and shortly afterwards a friend taught me some quilting basics.
Ive been quilting for about 12 years, I think. So it's not just inexperience getting in the way. I yearn to get away from traditional quilts and to find a way of using my quilting skills to produce original work that expresses something of me. But that's as far as it goes, as I have no more specific ideas about what or how that expression might be. I think I have an artist's eye: I really look at things, am aware as I move through the world, respond to colour, beauty, pattern - but don't have the first idea what to do with that. I can't draw or paint, so it has to be another, more abstract medium.
Ive been using a design wall (well, my bed, as I don't have a big-enough wall in my small house!) for this quilt, and thought I was doing ok with it. Then as I tried to show a selection of my chosen fabrics to my sister, I could almost physically feel my confidence and excitement draining away - suddenly it didn't look that good at all - just a haphazard mess. Her suggestion could be used to pull it all together, but I so wanted this to be my own original project and it wouldn't feel like that if I knew the only reason it looked good was because of someone else's idea.
The comment about being an introvert resonated, because I'm one, and I did think last night that maybe what I've learned is to work as you describe, and keep it all to myself till its done. And yet.... In spite of what's happened this time, sometimes it's fun to share (though I've always done most of my quilting alone, as there's no suitable group anywhere nearby).
I think id really like to do a course that encourages exploration of creativity through teaching techniques and suggesting exercises. My sister has done a City and Guildscourse that included lots of that. I spent ages on Google last night to see if I could find a distance learning/online course like that, but, quite surprisingly, I drew a blank.
I mustn't forget to catch my train....!