I don't know why you would have associated an STD with HIV in the first place. They are NOT mutually exclusive. Tons of people have STDs, and no trace of HIV. There are more people out there with an STD only than there are people with an STD plus HIV, or HIV alone. I think you have been traumatized to the point that you have a fear of rejection from the opposite gender because of your past. There is no reason to feel guilt. Again, lots of people have had trysts and one night stands. You learn from your mistakes and move on, and don't do it again. As others have pointed out, there is no need to mention to partners a past STD that has been cured. Do you tell partners that you had an ulcer, an appendectomy or a yeast infection? Maybe, if you guys are discussing past medical history for some reason like "oh, I had my appendix taken out at 10, it really hurt" and you say "oh yes, I went through that as well" otherwise, it's not really the norm to discuss these things with partners.
I think your reason to sleep with this man was wanting revenge on the fact that your fiancé left you for another woman so you felt you were getting back at him by doing the same thing, or maybe you wanted to feel what it was like to be on the other side of things (the homewrecker side). See it as sexual exploration, like some woman trying out a one night stand for the first time. You obviously realized this was not for you, you do not like it, and do not want to ever do that again, which is good, it means you have a conscience and a moral compass, so just don't do it. Again, lots of people have something in their past to be ashamed of, it makes us grow as people. I would not take the cheat back though (your ex fiancé), what makes you think he won't keep with his behavior? If he is doing this while engaged, what makes you think getting married would do to change that? Stop being so concerned about what he, or anyone else, may think, and work on yourself, your own happiness and peace of mind.
Yes, you probably do need friends, based on what you said. Why not get into some hobbies? That would be a great way to make some friends. I live alone too, have gotten myself out of some poor choices, and I am fine. Being alone is not the same as being lonely. You could be living with your ex-fiancé and feeling lonely because he is cheating on you, for example. I feel quite fulfilled with my close friendships, my hobbies, and keeping busy. Read some books, catch up on some movies or TV shows you may have wanted to check out.
Do you have family nearby? I would feel more comfortable talking this over with family than a friend, although seeing a therapist would probably be the best solution, to get over your fears and disgust for yourself, and to help you get out of this rut. Don't be so hard on yourself. I would visit my friend but not to dump on her, just to have a fun girls' night out, or to visit a spa, maybe do a road trip...something fun to get your mind off of thinking about these men and the trich. The more you think of this and keep replaying it in your mind, the harder it will be for you to move on.