In a medium saucepan, combine one can of coconut milk with half a can of water. Heat over medium flame until it reaches a gentle boil, stirring occasionally. Once boiling, mix in sweetened condensed milk to taste, ensuring the mixture is smooth. Remove from heat and allow to cool.

In a large pot, bring about four cups of water to a rolling boil. Carefully add in the sago pearls and reduce heat to low, allowing them to simmer for 10 minutes. Remove from heat, cover, and let sit for an additional 10 minutes. The pearls should appear translucent and chewy; drain and rinse them under cold water.

In a spacious mixing bowl, combine the cooled coconut milk mixture, diced ripe mangoes, cooked sago pearls, and coconut jelly cut into small pieces. Stir gently but thoroughly, letting the flavors meld.

Transfer the Mango Sago mixture into individual serving bowls or a large dish, then cover with plastic wrap. Refrigerate for at least 3 hours before serving.