We tell ourselves all kinds of unhelpful things every day. For example, I'm ugly, I'm never going to pass that exam, and I'm useless. Paradigm has a useful article on how to change negative self-talk. They suggest keeping a thought journal and write down your negative thoughts about yourself. I would also suggest that you replace them with a positive statement about yourself. For example, I'm beautiful in my own way, I will work hard to pass this exam, and I am capable. It takes a lot of practice replacing negative self-talk with positive self-talk but it is well worth the effort.