Reconstruct the moment
Identify yourself, the approximate time, what they did, and the situation around it. Be careful with private health, financial, family, or workplace details in an unexpected message.
Describe the result they could not see
Explain the later opportunity, habit, safety, confidence, or decision that followed. Keep the credit accurate rather than assigning them responsibility for your whole life.
Ask for nothing in return
Say explicitly that no response is needed. Do not turn gratitude into a request for renewed support, forgiveness, or a meeting unless the relationship naturally opens that door.