I think this is common. A part of you might be a little sad she has a boyfriend because it might effect your relationship with her. For example, she might not have as much time to spend with you, as she once did.

I think it is good that you’re insightful and able to identify your thoughts and try to change them to positive ones. We all have negative thoughts about ourselves and others- even people we care about. Being able to recognize these thoughts, try to understand where and why we are feeling them, and make efforts at countering them is the best we can do in my opinion.