He slept with a girl that had a STD and didn't tell me HELP?!

Okay me and my boyfriend have been going out for a while. Well we got into a huge arguement the day before valentines day because i found out he had sex with a girl that is known for messing with every guy and has a STD(Has had different types and will openly admit it) and now has herpes. Then didn't tell me about it before we had sex, even though he said he used a condom that still scared me because HE DIDNT TELL ME! and admitted he thought he had something for a while. I flipped out about it and he flipped back on me and told me i was being a ******* *****. An just need to trust him because he didn't give me anything...then was like i need to learn how to trust him if i want to be with him because we been together for 2 1/2 years and she was just a girl he was messing with when we broke up once (for like a week) but im still beyond nervous because that was my first time ever having sex and to find that out scares me! He called me crying last night trying to have me talk to him and work it out but im like what the heck !
i'm 17 by the way......advice please
am i over reacting?
what should i do????