First off, just know that it's normal to be afraid. If you are a smart individual that is used to critical thinking, you have figured out that street fights can be dangerous and you can get seriously hurt.
Second, sometimes it's better to run than to fight. Even the toughest guys can't fight multiple people at once (yes, I know sometimes one tough dude will beat up multiple opponents and sometimes you can't run and you have to fight, but these types of fights usually mean you're going to take a beating).
Third, try to figure out what you are mostly afraid of. Most people are not afraid of getting hurt, they are mostly afraid of losing in front of others and being dominated by someone else. I hated fighting in front of girls, but had no fear if it was just in front of my friends (who incidentally I knew had my back)
Fourth, if standing up to bullies physically is something you want to do, then you need to rehearse it. Training and sparring is how you can do this and it will build your confidence. The first time you go a whole round with someone and know that you took care of yourself in there, your confidence will sky-rocket.