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To be honest, that seedling looks pretty rough. I suspect that even if you kept it above 50F from now on it will rapidly decline.However, I'm happy for you on your first soursop flower! Mine also flowered for the first time this year, but fruit did not set. I'm hopeful it will hold fruit next year.
My soursop is in ground 8 years, flowers a LOT and only one fruit so far. Grrr!