speaker concluding that these other experiences, these mistakes in love (or "worse essays" -- essays means attempts), only proved to the speaker that the young man is his "best of love."
In the third quatrain, the speaker pledges that his experiences are over. He will never again try to love someone else ("newer proof"). Instead, he will be like a prisoner (suggested by the word "confined" in line 12) to the young man, who is referred to as the "older friend" and a "god in love."
In the closing lines, the couplet, the speaker asks the young man to "welcome" him. He is asking the young man to accept his mistakes and believe that his love from that time on will belong only to the young man.