Hi. I'm not in a relationship, but I am worried that when I get into one, I will be cheated on. Is this a normal feeling to have about relationships? Why do I feel this way?

Hi. I'm slightly insecure about my size...down there. I am 18, and I'm probably about average (I never had it measured though). I have never been in a relationship, and am kind of worried that when I start one, I won't be enough to satisfy my partner. Is there any advice you could give me?