I think you should go see a doctor and get tested for whatever you are worried about, and treat it if you have caught something. Also, I think you are very worried and scared, more than you really need to be, because you don't know much about these things. My advice (worth its weight in gold!) is to try to relax, and educate yourself, and maybe find some people whom you trust to talk with about it. I realize that last thing might be hard (I'm guessing you are not out as gay?) but it can really help.
Here's how I think of it: All these STDs are just diseases like any other ones. Some of them are uncomfortable and some can kill you, but the same is true of all the other diseases you can catch in other ways. So there's no need to freak out, just evaluate the risk/reward equation and decide what is right for YOU. And I don't believe you have to feel guilty about passing diseases on to another person sexually, unless you do it deliberately and deceptively. The world is a dangerous place, and they decided to have sex with you.