You have a phobia. You're like people who are afraid to fly or drive on the freeway. If this is your only phobia and you don't have lots of things that you respond to with this level of anxiety, you should be able to deal with it without a huge amount of trouble by getting therapy. You might even talk yourself out of it by getting some books on sexual health and learning how this stuff actually works and how to protect yourself, which might demystify this fear you have. We do lots of dangerous things every day, and the best we can do is mitigate the danger as best we can knowing there's no 100% safety out there. We do this every time we drive a car or climb a ladder. Life is full of risk. But when you overwhelm yourself with fear, you become frozen and don't function and that's worse than what you're trying to avoid, so that's what you need to work on.
I think that is great you booked therapy. Dive into it. Sometimes facing our fear is the way to conquer it even if uncomfortable. STD's aren't great but generally don't kill you. Most you resolve and recover from completely and only a couple leave a chronic situation to deal with that is now managed with modern medicine. You can prevent them with safe sex pretty well or by being in a monogamous, trusting relationship and periodic std testing for good measure. Talk through these fears with a psychologist and why you have them and how you can resolve them so you can have a normal sex life without freaking out.