Unless you want to make yourself believe that your ex-girlfriend chose to date you knowing full well that you were ugly, it is highly unlikely that you are hideous. The fact is, girls have a much easier time finding boyfriends than guys have finding girlfriends.
Girls are pursued by men. Men, for the most part, aren’t pursued by women. She didn’t have to do much of anything to find this new boyfriend. He approached her. Wooed her. Proposed an exclusive relationship to her. He was the one that did the lion’s share of the work. With these advantages, it’s no wonder that your ex-girlfriend has easily found someone else.
As a guy, you have to put in more work to find a new girlfriend. You are better off not focusing on what she is doing. Rather, look to your life and where you can make positive changes. If you gained weight while you were dating her, thanks to complacency, get into the gym and start working out so that you lose the weight and become more physically attractive to women.
Also, expand your social life. Get into the habit of going out to social events more often. This gives you exposure to multiple women at a time which increases the likelihood that one or more of them takes an interest in you.