All of Scripture has to be incorporated to arrive at a right conclusion. The Lord’s prayer does not mention “if”.
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2*And forgive us our debts,
As we also have forgiven our debtors;
For if you forgive men their trespasses, your heavenly Father also will forgive you; 15*but if you do not forgive men their trespasses, neither will your Father forgive your trespasses. Matthew 6:14–15
Neither do other passages that speak of forgiveness. Nothing about repentance here…
21
Then Peter came up and said to him, “Lord, how often shall my brother sin against me, and I forgive him? As many as seven times?” 22Jesus said to him, “I do not say to you seven times, but seventy times seven.
“unless you forgive your brother in your heart” is consistent. We are to leave the sacrifice on the altar, go, and be reconciled, then return to the altar.
I will concede that there are times when reconciliation is not possible. Both parties must be willing for reconciliation to happen. But that does not absolve us from the commandment to forgive.
5*And whenever you stand praying, forgive, if you have anything against any one; so that your Father also who is in heaven may forgive you your trespasses.” Mark 11:25
“Judge not, and you will not be judged; condemn not, and you will not be condemned; forgive, and you will be forgiven; Luke 6:37–38
Nothing about waiting for repentance here either…
47*Therefore I tell you, her sins, which are many, are forgiven, for she loved much; but he who is forgiven little, loves little.” Luke 7:47–48
Forgiving others opens us to love much. Waiting until they repent keeps us in “love little”.
31
Let all bitterness and wrath and anger and clamor and slander be put away from you, with all malice, 32and be kind to one another, tenderhearted, forgiving one another, as God in Christ forgave you. Ephesians 4:31–32
Forgiveness frees us from bitterness, wrath, anger, resentment and a multitude of bad fruits. God did not wait until we repented before He gave His life to pay for our sins. Neither do we need to wait until others repent to forgive them.
Maybe there is some reason that you must hang on to this “if” but it cannot be applied across the board. It is clear that Jesus did not wait for those who crucified him to repent. Waiting for others to repent will just stunt our growth.