You've been on MedHelp long enough to know what isn't a risk. If these types of encounters make you nervous then I suggest that you stop having them or seek professional mental help to help you accept that they are no risk.
I just can't stop second guessing myself. All i think about is "what if the condom slipped" or "what if i missed a really crucial event that may have created a risk.
Well as you have read hundreds of threads you will have seen the advise has never and will never change. HIV is not transmitted via oral sex, its that simple so i dont see how hard it is to understand?
Dr Jose from HIV internation, the doctors forum has done a very detailed blog regarding this subject, so have a read of that to help yourself.
How to overcome them, well dont have unprotected oral and then their is no reason at all you could even think about HIV, not that your at risk anyway, but simple logic and medical facts do no reassure you.