Nothing can stop me or nothing could stop me?

“I will keep trying. Nothing can stop me” or “I will keep trying. Nothing could stop me?”

Which one is correct? I find it extremely hard to choose.

The former is a somewhat stronger sentiment. “Nothing could stop me” is closer to “Nothing would conceivably be able to stop me.”

Your question mark should appear outside the quote marks, as the quote is not interrogative. Although I recognize this is not a full sentence, that is still unacceptable.

Thank you so much. You are a great coach. You even told me something about how to use question marks. Thanks again.