Will is apologising for being a naughty boy and pleading with his old flame to take him back. Beware of the most obvious meaning, he might just have been neglecting an old friend, not a lover in the sexual sense.
Here's a paraphrase I wrote in modern colloquial speech with a few ambiguous words trying to cover the possibilities. Forgive me but I didn't go so far as to make it into a sonnet with proper metre and rhyme.
I regret that I've fooled around with others
And made a fool of myself in public,
Been false to my principles, betrayed our love,
And added new infidelities to my list of offences.
I have ignored what I knew to be the truth
But I swear that realising all these things
Has reformed me, made my heart young again,
And proved to me that you really are the best.
I'm finished with all that: accept my eternal devotion,
I'll never have intercourse with anyone else again,
Young though they be, to test you, my old flame.
You're my love god and you've captured me.
Please take me back, the next-best thing to heaven
Is your company, your purity and your devotion.
The Proposal by Carl Rudolph Sohn, C.E. 1881.
The Proposal by John Pettie (1839 - 1893)
The Proposal by Laslett John Pott (1837-1898)
Alas, 'tis true I have gone here and there
And made myself a motley to the view,
Gored mine own thoughts, sold cheap what is most dear,
Made old offences of affections new;
Most true it is that I have look'd on truth
Askance and strangely: but, by all above,
These blenches gave my heart another youth,
And worse essays proved thee my best of love.
Now all is done, have what shall have no end:
Mine appetite I never more will grind
On newer proof, to try an older friend,
A god in love, to whom I am confined.
Then give me welcome, next my heaven the best,
Even to thy pure and most most loving breast.