Here's a prayer titled "Assurance" from "The Valley of Vision: A collection of Puritan Prayers & Devotions" edited by Arthur Bennett (pages 92-93):
I am loved with everlasting love,
clothed in eternal righteousness,
my peace flowing like a river,
my comforts many and large,
my joy and triumph unutterable,
my soul lively with a knowledge of salvation,
my sense of justification unclouded.
I have scarce anything to pray for;
Jesus smiles upon my soul as a ray of heaven
and my supplications are swallowed up in praise.
How sweet is the glorious doctrine of election
when based upon thy Word
and wrought inwardly within the soul!
I bless thee that thou wilt keep the sinner
thou hast loved,
and hast engaged that he will not forsake thee,
else I would never get to heaven.
I wrong the work of grace in my heart
if I deny my new nature and my eternal life.
If Jesus were not my righteousness and redemption,
I would sink into nethermost hell
by my misdoings, shortcomings, unbelief, unlove;
If Jesus were not by the power of his Spirit
there is no sin I should not commit.
O when shall I have his mind!
when shall I be conformed to his image?
All the good things of life are less than nothing
when compared with his love,
and with one glimpse of thy electing favour.
All the treasures of a million worlds could not
make me richer, happier, more contented,
for his unsearchable riches are mine.
One moment of communion with him, one view
of his grace,
is ineffable, inestimable.
But O God, I could not long after thy presence
if I did not know the sweetness of it;
And such I could not know except by thy Spirit
in my heart, nor love thee at all unless thou didst
I bless thee for the covenant of grace.