Combine lemon juice, water, gelatin, and cornstarch in a saucepan. Heat slowly, stirring constantly, until gelatin and cornstarch dissolve. Add fruit to gelatin mixture. Heat to boiling point over low heat, stirring constantly. Boil berries 3 minutes (other fruit may take longer). Remove from heat and add sugar substitute. Pour into heat proof jars and cover. Let set at room temperature until cooled. Store in refrigerator or freezer.