Well, you're 18, right? In most countries, that makes you an adult. I know that even when you're legally an adult, at 18, most of the time, you're still dependent on your parents for a lot of things, including money.
And I don't know what you've done sexually, but what you are describing isn't anything like an STD, and things go wrong with your genitals that have nothing to do with sex. Women (and men) get breast cancer, right? That's not sexual.
Even really religious parents have to know that things unrelated to sex can happen to your body. Virgin men can get testicular cancer (which I don't think you have, but you are in the age range for it). Men can get urinary tract infections without ever having sex.
I understand that one of your symptoms is painful erections, but can you skip that symptom when talking to your parents? If you've had sex, you don't need to tell them. Do you need their permission to see a doctor where you are? (And you don't need to tell us where you are.)
Not knowing where you are, I don't know how your health care system works. If you have any family planning clinics where you are, you can call them and ask about that.
I'm not a deeply religious parent, but I'd definitely want to know if you were my kid. I'm sorry you're so afraid of your parents. Is there another trusted adult you can talk to? A friend or uncle or even a school nurse or counselor or something?