First, why are you having unprotected sex anyway? You're just setting yourself up for anxiety and for other consequences -- HIV isn't the only problem out there, as I'm sure you know. Second, it sounds like you're taking antibiotics without having anything that antibiotics treat, and you're possibly suffering some of the consequences of that -- getting sick repeatedly and developing immune system problems. Antibiotics kill of organisms that protect you from disease, so you only take them when absolutely necessary. If you had a successful root canal, the major infection is gone, and what's left is sealed in the tooth and probably can't be reached by antibiotics. Sinusitis doesn't mean anything -- it just means you had an inflamed sinus, but doesn't say why -- most likely it's just allergies, which will get worse the more you take antibiotics. I'd go to a good health food store and buy the best probiotic found in the refrigerated section of the supplement department. You can also benefit from eating fermented and cultured vegetables to replenish beneficial organisms. Lastly, have you seen a therapist who specializes in anxiety treatment so this doesn't become a chronic problem, as it seems to be doing?
I think you definitely sound anxious and really hope that you see someone about your anxiety. Irrational fear is hard to live with as you are finding. Talk to your doctor about it.
"Later that day I got sick ...." Are you aware you don't get any symptoms that fast?