I think you ask great questions here, along with making truthful statements. It would 'seem', in a way, that Matthew 6 is not plain in its meaning.
First we have to lay out the obvious facts. Jesus never lied. Some things of Jesus were 'hard' to understand. We have more than one instance of this 'forgiving'.
Mat 6:14-15
For if you forgive others their trespasses, your heavenly Father will also forgive you, but if you do not forgive others their trespasses, neither will your Father forgive your trespasses.
Mar 11:25
And whenever you stand praying, forgive, if you have anything against anyone, so that your Father also who is in heaven may forgive you your trespasses.
Luk 6:37
Judge not, and you will not be judged; condemn not, and you will not be condemned; forgive, and you will be forgiven;
Luk 17:3-4
Pay attention to yourselves! If your brother sins, rebuke him, and if he repents, forgive him, and if he sins against you seven times in the day, and turns to you seven times, saying, ‘I repent,' you must forgive him.
So three out of the 4 Gospels have this statement in them. Must mean that its serious.(not that one instance is not enough) What they do, however, is show us different angles of what is being said.
We know that Jesus shed blood is the payment for the sins of the world - to all who will believe. Forgiveness has to be received - it is not forced on anyone.
Here is where we understand. Would God command us to do anything He would not do? Of course not. When we repent, He forgives us. If we are in Christ, and God's Spirit lives in us and leads us - would God not lead us to forgive when someone repents?
If we choose to ignore the Spirit of God leading us to forgive - do we believe God? Would you classify that as unbelief? Can you only believe some of what God says and it be complete belief? Do we think God only requires partial belief?
Point is, when we come to understand God's forgiveness and how we receive it, then it is a natural process to give it also. If we do not give forgiveness, then there is an issue - either we do not believe God, or we only believe what we want of Him. Neither of those is true faith. We have to understand it is a sin to withhold forgiveness. A person in Christ cannot continue in sin.
There are good reasons
why God wants us to forgive, but that would be another topic.