At 15, it is normal to be ambivalent about wanting to get involved romantically. Most kids start wanting to date but are also at least a little afraid of it. Showing interest in someone means risking rejection. Getting involved means making important decisions about how much of yourself to share with another person. It means figuring out how much to trust someone and making decisions about physical intimacy and sex. It’s likely that you, like lots of kids your age, are avoiding having to deal with all of that by finding reasons to dislike the person you were initially interested in.
It might help if you figure out for yourself what you are and are not ready for in terms of a relationship. If you are clear about your own boundaries it will be easier to maintain them if you feel pushed by another person to do things you don’t want to do. Also — take it slow. It’s normal for someone your age to pretty much stick to doing things with a group of friends for awhile. That lets you learn more about relationships — and have some fun — without having to make any commitment to a particular person.
I wish you well.