How often should one get tested for an STD?

I am 17 years old and I am gay. I have only had twice with the same person, and it was this year. Before doing so, I asked him if he was clean. He said he got tested in March, and came back clean. (We did it in May). So being a virgin with hormones out of control we had unprotected both times. Well, after the second time, I asked him to go get tested because I became a little paranoid. He came back positive for a treatable/curable STD, syphilis. At first he told me chlamydia, for that was what he was told. So when he goes back to get the medicine, they tell him its syphilis, and give him medicine for it. He also told me that the doctors at the clinic said that the levels of it were very low so it was least likely to have been transmitted. I go get tested, waiting and being paranoid, and came back clean. (I got tested about 4 weeks after my first time.) NOTE: He cheated on me with another guy, he said it was only , but I doubt it. AND, because of this incident, I have high stress levels, paranoid about my health beyond natural means, I lost 8 pounds in a short time from doing nothing. So, my question is, what if the test didn't see anything YET? What if it was wrong? How often should one get tested after such an incident?