Competitive rhythmic gymnastics is just as tough as artistic. Stretching does take up a lot of practice time, but you need not just flexibility but also apparatus skills, dance skills, body skills, and expressiveness/performance skills. In order to be competitive, you will need to learn and practice all of those things. You might want to attend another practice class where you could try some of the apparatus. You could also try to see if there are any competitions happening in your area that you could go watch, so that you could get a better sense of the types of routines that gymnasts your age are competing.
My daughter competes in both artistic and rhythmic Xcel programs so I'd be happy to answer any other questions you have.