Scenario: His penis was inside you without a condom either for a moment or the entire time and you don't know his STI status.
Animal Advice: Don't beat yourself up.
Action: Instead of sitting around Googling "symptoms of AIDS," get your ass to the doctor or the clinic and get a full battery of STI tests. Because whatever catastrophic scenario you're imagining is probably worse than the reality. But you have to face the reality. No avoidance behavior! When your test results come back (all negative fingers crossed!) then you can forgive yourself for the mistake. And decide not to put yourself through the stress of that again.