A lot of secondaries are asking to mention a time you had to overcome an obstacle or when you faced failure. I recall during my junior year I tried to joggle many ECs at once, I had 2 research positions, was volunteering, trying to maintain a decent GPA, a social life, clubs/activities, etc. Unfortunately I was let go by one of my PIs since I was not able to dedicate time. Would this be okay to write about or does that just make me look bad?
Meh, two problems. For one, it's probably the most common answer medical school adcoms here to the challenge question. Try to stand out and talk about failure in a sport or team activity. Dig deep, there are likely several everyone has. It doesn't have to be like the one the poster above mentions and because you don't have those experiences doesn't make you a less capable person as struggles can be relative and if you respond effectively to small struggles and learn from that, that still shows proactiveness and maturity. If you end up sticking to this script, that's OK but then at least work on the way you describe it. I would not mention being let go for instance and would introspect deeper into what caused you to make so many commitments and how you avoid these situations in the future to maximize your efficiency.
For secondaries, prioritize
1.) Not writing anything that makes them question your character/judgment/competency. There are a million other things that adcoms look at other than secondaries and most read similarly. It would be a shame for this to stick out in a bad way.
2.) Not far behind is TIMING. I know this sounds extreme, but I would literally prioritize 1-2 day turn-around for secondaries. I know it's hard because they come in waves, but stay on top of it and don't fall behind. Work hard and take the day to compose a thoughtful answers, but then churn them out. Pretend it's like it's the written section of a standardized exam.
3.) Spelling mistakes. Take time to proof-read aloud. Demonstrates carelessness on your part. Take a break after composing them and proofread again in the morning. Print the paper out and go through it with a red pen if needed. Watch out for overproof reading too which can create sentence structure errors.
4.) Narrative quality, standing out, all the fancy stuff. Either you have a unique narrative, or you're an upper middle class, college junior from the suburbs who does the same things do to get into medical school as 60% of medical school applicants. Don't try to push the narrative if you don't a great one and just focus on the introspective aspect and English 101.