RIGHT Firstly: BREATHE!!!
There is absolutely no reason, from what you have put on here to suspect an STD, no reason whatsoever! As others have said, these routine tests will test for things like thrush, chlamidia, PID (pelvic inflammatory disease - not so bad as it sounds) Urine infections etc. All things that can make you feel pretty shitty.
You need to go back to your doctor and talk to them, or a nurse, anyone. Please do this. You sound like i did one day when i had an absolute meltdown. Fortunately for me it was the begning of me getting the help i needed.
You sound completely and utterly over the top anxious about this - i know exactly how you feel, i was in my doctors once, hysterical because i thought my tonsils were tumours. Once the anxiety kicks in, rational thought goes out of the window.
You probably feel run down and unwell because of your anxiety and it has caused you to be depressed. You NEED some treatment for this otherwise you are going to get worse. Sorry to be blunt, but ive been there and honestly, i could have written this post 6 months ago.
You are NOT a bad person, you deserve to be happy, your depression is what is making you think otherwise. Im not a doctor, but i can say this because this is exactly how i felt. In fact it was a thread a posted on here ages ago that said something along the lines of me not deserving my family that prompted me to get help. Ive not looked back, OK so im not 100% ive got a LONG way to go, but im getting there. Please dont make yourself suffer this anymore - go to the doctors, just turn up if you cant get an appointment and insist on seeing someone, jump up and down, shout, cry - whatever just do it. If there are no doctors appointments and you have a minor injury unit, go and take a ticket and tell someone there, thats what i did, funnily enough i was given an appointment within the hour!