If the Lord has asked you to fast a partcular way, then do it. We cannot control another person. We can only be obedient to do what God has instructed us to do.I encourage you to search the scriptures for verses related to the situation and keep searching. Put the person’s name in them and pray them for that person. Ask the Lord to work on the hearts of all involved. I often ask the Lord to work on my heart too. It is not the sugar that is as important as it is the attitude of the heart. Is there bitterness of the heart somewhere? That may be what the Lord is telling you.
Heb 12: 15 says “Looking diligently lest any man fail of the grace of God; lest any root of bitterness springing up trouble you, and thereby many be defiled;” You used the word bitterness—If there is bitterness there is a root.If there is, the Lord knows the root.Ask the Lord to remove that root through His Word and His Love. Matt 6:12 is also important. To remove a root there has to be forgiveness. The presence of God’s Love can bring forgiveness.
Possible intercessory prayer focus might be for God’s word to work on the bitterness, removing the root of bitterness, forgiveness, and God’s Love.
Remember, God’s Word is very powerful and will accomplish the purpose to which it is sent. Is 55:11 says: “So shall my word be that goeth forth out of my mouth: it shall not return unto me void, but it shall accomplish that which I please, and it shall prosper in the thing whereto I sent it.” Don’t be afraid to pray God’s Word back to Him in intercession. God accomplishes His Word—especially when it comes from our heart.
I hope the above helps.
May the Lord give you direction and wisdom as you pray. May Lord guide your heart with His Wisdom and Direction. In Jesus’ Name, Amen