How do I believe in myself more while in a relationship?

It's the first time I've been in a relationship. So everything is new to me. I don't feel like I'm pretty enough to have a romantic relationship with any guy. But somehow, I got a good boyfriend who always cheers me up when I'm feeling down. When I'm around him, I can't feel like I don't deserve to be with him because he would always kiss me if I feel this way. This is normal in a relationship but I can't get accustom to it, so I would get really nervous, shy, and my heart would skip a beat when he does it. Most of the time he does it very suddenly, and this cause me to have a stronger shy reaction.

The other day, we were going out to see a movie and my friends wanted to help me get dress up. They pick out something that I never wear before, a short dress. I don't wear short dresses or skirts because I always thought that I look ugly in them. After I put it on, I did look ugly. My friends said I look cute and that my boyfriend will think the same, but I refuse to show him. My friend bring him to show him how I look in it. He ask my friends to leave the room. After they left, he ask me why I don't want to wear the dress? I didn't want to tell him how I feel because he might kiss me and make me get really shy and nervous. So I told him that it didn't fit me. He said that's not true. He step closer to me and I step backwards. He told me I look very cute in the dress. I stop him from getting closer by saying I'll wear it. He smile and asked what make me change my mind? I didn't answer him.

How do I believe in myself more while in a relationship?
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