If the event and made me regret the most is the loss of someone we love can never return. That is, I'm sorry that my father died while I was six years old, and I'm sorry that my grandmother had died.
If the event was terrible and made me most sad is. The loss of a loved one, never to return. That is, I regret that my father died when I was six years old and I regret that my grandmother died.
If the mentioned events that bad and I regret most is Losing your loved ones never to return. The I'm sorry my father passed away while I was 6 years. And I'm sorry that my grandmother died.