There's no reason to scare yourself or work yourself up about STDs and such, its entirely possible to have safe sex, sure even a condom can't provide you with 100% protection, it can be faulty, misapplied, whatever, but the chance is minuscule and the risk insofar as it exists only applies if you're having sex with someone who has an STD, which most people don't, so its incredibly silly to get worked up about it. You can always get tested and insist that others get tested and only have sex with people who have no STDs and use protection just to be sure and you'll be fine.
Sure there's always the possibility of a test being wrong or of someone out of malevolence lying to you and that of a condom or any other form of protection being faulty but equally anytime you walk outside you risk being struck by lightning and any night someone might break into your house and murder you, these things do happen and there isn't much of a reasonable way to prevent them 100%, yet we can't live in constant fear of these and let it guide our daily conduct. Take reasonable safety precautions and you'll be fine.
Even in the extremity that you are unfortunate enough to catch an STD we thankfully live in an era where these things are treatable and won't ruin your life. Even in the worst case scenario of catching HIV, it is no longer the death sentence we still fear it as, sure its not nice to have to take pills for the rest of your life but as long as you do you can live an entirely normal and healthy life which is miraculous and we should all be glad for it and that most likely scientists will figure out a complete cure for it within the next few years.
So yeah, don't be scared, be safe and have fun, ideally with a consistent caring partner you trust and get tested with regularly.