It is not true that the story has already occurred if it's being told to the reader. Even a past tense story can still be unfolding in real time; it's just that things are being told to the reader the second after they occur, rather than right as they occur, which would be present tense. The story turns and events can be just as surprising and unknown to the narrator as they are to the reader. If your story is told in the simple past overall, all action should be in past tense, but whenever your narrator adds commentary, which is merely spoken thought, even if your story is told from the distant future and looks back, tense can switch to present when what is said is still, typically or always true. When you don’t shift, it puts your narrator in the grave or creates strain or an untruth. Just remember to switch back for any action.
[e.g. I couldn’t stand cake… VERSUS I can’t stand cake, so as they brought out that hellish thing with its mountain of frosting, a snarl contorted the left side of my face.]
"I couldn’t" would mean it’s no longer true or the character is deceased. If the commentary is still true—Graduates from my college get key positions ...Valentine’s Day reminds me of the day my dad walked out on us… Mick’s Diner has the best burgers … She sucks the big one ...The Dallas Cowboys are America’s team—present is fine, then, it’s back to past tense for the action. Everything should sound natural and conversational from the point in time in which you’re telling it. People in real life do not think or say any present truth in past tense, even if they’re telling someone something that happened decades ago. Many writers struggle with this sense. They intuitively know they’re saying something incorrectly with a past tense verb when the said commentary is ever-true, so they plug in a present tense verb until some hired and well-meaning editor scratches it out with a red line and says, ‘Switch to past tense. Be consistent’.” They just shrug their shoulders and do what was suggested because a pro must know. No, you know. If it’s not action we’re talking about but commentary and a story-present-truth, regardless of its time frame, present is fine.
It's a fallacy when people say that stories in past tense are from some distant point in the future or that it has already happened. In your case with the flipped bird, in a story that has already occurred that she's telling, it should be past tense because it's action. The reason you are getting conflicting stories is because present tense is more common now, but it's not the only way to tell a first person story. Present tense is difficult to maintain and wield. And people using past tense feel that verb stain I mentioned with the commentary.
Just think: any action = past tense. Commentary, random tidbits, truth, audible or internal thoughts along the journey = are cool as present.