The Law says that we have to be perfect...that means to forgive perfectly. Jesus pointed that out because none of the people could achieve that. So He died on the cross and our sins are forgiven. Those who believe will have eternal life. Those who believe and are baptized are clothed in Jesus and that means that His perfection is ours and we lack nothing.
Where does the law say to "be perfect"? Not saying it doesn't, I'd just like to know where.
Jesus also says "be ye perfect".
...and at the same time "forgive us our sins, as we forgive those who sin against us"
So it seems to me that he expects perfection, recognizes that we will fail at times, and places our forgiveness squarely on whether we forgive others.
"forgive perfectly"
suggests that forgiveness can be partial or incomplete. Like maybe a 50% forgiveness or something. I've never heard of such a thing as it relates to a particular sin to be forgiven an individual.
And then there is this, which references your point:
Matt 26:28 For this is my blood of the new testament, which is shed for many for the remission of sins.
I've always found this curious because of the present tense in which Jesus speaks it. Not "will be shed" but "is shed". The bread also is not "will be my body" but instead "is my body", all of this taking place before the cross.