First, this forum is just for anxiety. No risk assessments here and you've had many regarding your fear of std's. I would consider working with a psychologist. You've been given the facts that oral sex while isn't no risk is certainly very low risk for std's and that in the absence of symptoms, you don't need to test or whatnot. But, if you aren't sure of your partner's status, use a barrier. Keep your activities to a specific partner that is tested along with yourself and don't put yourself i situations that aggravate your worries. But regardless, you are in a thought loop. To come out of the thought loop,, you break the pattern. Asking here or anywhere, researching, thinking, asking others for reassurance, etc. are all compulsions that go along with an intrusive thought. You have to stop the compulsion. Label it when you have it. Oh, this is just anxiety thoughts again. And anxiety lies and tries to trick you. Once you label it, be uncomfortable for a bit. Worst case scenario, std's are not really 'that' big of a deal. Either treatable or manageable. Then go find something else to do. Overthinking and analyzing is the hallmark of anxiety. Give it up.Talk to a psychologist and consider medication if they think that will help.
Medical anxiety is a real thing. It's anxiety, but transferred from some other aspect of your life. So, the person worries about their health instead, because that's more cut and dried.
The medical condition someone has anxiety about might give a clue to what they're worried about deep down. For example, it's pretty common for a woman to write into the DNA/Paternity community freaked out about maybe the wrong guy is the dad of their baby. And, they say that the wrong guy had sex with them weeks before they got pregnant, or something else makes it clear he really can't be the dad. But they still can't drop the fear. This happens to women who have other reasons the pregnancy is making them anxious. When the real cause of anxiety related to their pregnancy is deeper, scarier, feels more like survival, or just can't be sorted out, it's a lot easier for the anxiety to settle on the question of paternity. (It might be dire, but it's a simple yes or no in the end. Not like "What will my religious family say when they learn I'm pregnant and unmarried?" Anxiety like that can come out for weeks and weeks.)
So, if you are dealing with anxiety over STDs when realistically you don't have STDs, look to something else in your life that you might be anxious about, maybe relating to sexuality in some other way. Look at questions like (just for example) if you were raised to think sex was bad, or are worried that you won't perform well in bed, or are concerned women won't find you attractive enough, or think you need to perform like a porn star, or like that. There's probably a germ of your worries somewhere in there if you think about it. Then work out whether it's realistic to be fearful of that. If you can't get past the anxiety, see a counselor or therapist, not about being worried about STDs, but to get to the underlying cause of the anxiety.