Glad to be able to answer this question! A word of caution first, though--it's best not to ask about upcoming challenge pieces publically lest a judge should see what you've asked and be able to put two and two together, making your entry no longer anonymous. You've given very little away here, and our judges are scrupulous in their objectivity--but still, in the future, you might want to either ask those questions privately, or ask once the challenge is over. No big deal, truly, and don't fret about it.
Now, to your question.
Tense shifts are a common error in the writing challenge, and I think that many writers aren't even aware that they're shifting. It's very common to see something like:
"Are there any cookies left?" asked Ben. He opened all of the kitchen cupboards and even peered into the refrigerator.
Jan chews nervously and hides the box behind her back. "I don't think so," she said.
In this example, the first paragraph is in the past tense, with verbs like asked, opened, peered. The second paragraph veers into present tense, with verbs like chews and hides, but then it slips back into past tense (she said).
However, this doesn't seem to be your problem, as you seem to be aware of the difference in tenses in your entry. So I'd suggest a couple of solutions:
1. Since you know that they're not consistent, just pick the tense that works best in your entry and fix the ones that don't match.
That seems overly-simplistic, though, and I suspect that you've maybe used different tenses intentionally, perhaps for a specific effect. You said that you've written a true life story, so (I'm just guessing here) maybe your first paragraph is in present tense because you're describing yourself now, and the past tense parts are a flashback to then, and then the conclusion is back to events of now.
2. If that's the case, the only thing you have to do is to alert your reader that a flashback is about to happen. There are a number of ways you can do this--you could italicize the first and last paragraphs, or you could separate them from the body of the text with three asterisks, or you could just add an extra space before and after the center section.
I hope that answers your question! Feel free to follow up with a question of your own, as long as it doesn't reveal specific content of your entry.