I understand your fear, I myself had unprotected sex for the first time 2 summers ago. I found out the next day that the girl was known to sleep around, a lot. I was worried as soon as I found out. Now in my case, my fears are gone because the girl has since enlisted in the Navy where they do test for HIV and she's negative. But let me help you out...
This came from a medical website. "The actual chance of becoming infected with HIV during a single sexual experience — even with a partner who is known to be HIV-positive — is rather low: one study put the chance for a woman becoming infected by an HIV-positive male through vaginal sex at 1 in 1000."
So, the odds are low, but let's say you were infected. You say you'll feel like your life is over, perhaps you could do some research on the treatments they have these days. Typically I'd say don't google but knowing they have effective treatments may comfort you (even though you should be very confident that you aren't HIV+). HIV isn't what it was in the 80's. My uncle contracted HIV 18 years ago. I found out 3 years ago, my grandmother told me. Otherwise, I'd never have known. HIV+ people live pretty normal lives and long ones too.
The treatments they have now can actually reduce the virus so much that a patient can actually test negative. So know this, you more than likely do not have HIV. But if you did, it'd be okay!
Hope I helped! - AP