And forgive us our trespasses

Forgiveness
This is one effective way of keeping our hearts from growing bitter.

Eph 4:32 Be kind and compassionate to one another, forgiving each other, just as in Christ God forgave you.

We will forever be in need of forgiveness. First from God who bought us at a price and continues to forgive us since we are always in sin one way or another. we sin even in ignorance .
Rom 3:23 for all have sinned and fall short of the glory of God

God reminds us that we are capable of hurting others. We sin against Him by hurting people.
Matt 25:40 “The King will reply, ‘Truly I tell you, whatever you did for one of the least of these brothers and sisters of mine, you did for me.’

This knowledge, 'that we will also need forgiveness', should help us be merciful to others who offend us.

Matt 18:21-22 21. Then Peter came to Jesus and asked, “Lord, how many times shall I forgive my brother or sister who sins against me? Up to seven times?” 22 Jesus answered, “I tell you, not seven times, but seventy-seven times.[a]

Peter asks Jesus how many times he should forgive. In fact it seems the tolerance level was 7. This sounds awesome when we assume we are not the offenders. But when we put ourselves in the offender's shoe, we are inclined to want forgiveness so many times that we hope the other party does not remember our past offences.
Luke 6:31 Do to others as you would have them do to you.

If it is forgiveness, then extend it in the measure you would love to experience.

Matt 18:22 Jesus tells Peter that he should forgive about 490 times. I am persuaded that Christ is talking of this count as a 'no count'. Saying that as many times as you are offended forgive.

Reason is: 
Matt 18:35“This is how my heavenly Father will treat each of you unless you forgive your brother or sister from your heart.” (The Parable of the Unmerciful Servant)

Jesus notes that unless we forgive from the heart those who have wronged us, God will call us to account for our sins.
In the Lord's prayer, "Matt 6:12 And forgive us our debts, as we also have forgiven our debtors," -We ask that God forgives us just as we forgive those who have wronged us.

God is showing us how forgiveness is an acknowledgement that we are also potential offenders and that we are indirectly asking God and man to forgive us when we offend them. We will surely need forgiveness.

The weight of the offence might not be same but the need to be forgiven and forgive will always be there.

Matt 18:35 says we should forgive from the heart. I believe the heart is where pain resides. When we are offended and hurting, we store these experiences in the heart.

Looking at this truth, we should release the offender as though we needed the forgiveness.

Prov 4:23 Above all else, guard your heart, for everything you do flows from it.

One way of guarding our hearts is by forgiving people.We should not carry any hurt or bitterness, vengeance desire or evil thoughts in our hearts.

Prov 4:23 Be careful how you think; your life is shaped by your thoughts.(GNT) We don't want to live our lives shaped  by sin.

Forgiveness keeps bitterness at bay and humility closer.