What's he saying?
"Alas! 'tis true, I have gone here and there, / And made my self a motley to the view,"
I have been unfaithful to you, and have made a fool of myself in front of everyone,
"Gored mine own thoughts, sold cheap what is most dear, / Made old offences of affections new;"
I have sullied and cheapened my thoughts, which are so precious to me, and habitually took part in new love affairs;
"Most true it is, that I have looked on truth / Askance and strangely; but, by all above,"
I have beheld love, sincerity, and honesty with suspicion, as if they were strangers, but I swear now,
"These blenches gave my heart another youth, / And worse essays proved thee my best of love."
These instances in which I have turned away from you have rejuvenated me, and proved to me that you're better than the other love affairs.
"Now all is done, have what shall have no end: / Mine appetite I never more will grind"
Now that I'm finished with those escapades, take my endless love: I won't waste my sexual desires
"On newer proof, to try an older friend, / A god in love, to whom I am confined."
On younger people, to cause you, to whom I am loyal, any pain.
"Then give me welcome, next my heaven the best, / Even to thy pure and most most loving breast."
Take me back, for to be with you is second only to being in heaven.
Why is he saying it?
Sonnet 110 is a continuation of Sonnet 109, which established the poet's return to the fair lord after having been unfaithful. In Sonnet 109, the speaker vows to "bring water for my stain," or cleanse the "stain" on the purity of his love for the fair lord with his own tears. Both sonnets together are an apology for philandering. The idea of being welcomed back, put forth in the final couplet ("Then give me welcome") echoes the previous sonnet: "...if I have ranged, / Like him that travels, I return again."
The immorality discussed in this sonnet is tied to Shakespeare's occupation as a playwright; this and the following sonnet take a rueful approach to that station in life. Although wealthy, dignified people attended plays, and though playwrights were admired to a degree, a career in the theater was linked to loose morals. Specifically, in line 2 the speaker admits to having "made my self a motley to the view." The term "motley" refers to either playing a fool, or the clothes worn by the fool, which were patchwork. In Sonnet 111, he refers to "public means which public manners breeds," in effect blaming his behavior on a life in the theater.
The meaning of line 3, "Gored mine own thoughts, sold cheap what is most dear," suggests a physical and emotional unfaithfulness. The word "gored" implies an injury from the horn of an animal, and also calls to mind the dried blood and guts of the noun form, "gore." The idea of selling something "most dear" for a cheap price is likely figurative, though it could refer to the speaker's prostitution; either sexual prostitution or the figurative prostitution of his "own thoughts," dedicating his time to thinking about other love affairs.
In lines 10-11, the speaker declares, "Mine appetite I never more will grind / On newer proof," referring to the satisfaction of his sexual urges. The word "grind" appears in five other instances in Shakespeare, all of which are descriptions of unpleasant physical experiences. Here, it calls to mind the image of sharpening a blade on a grindstone to make it keener (like an appetite). The idea of having a keen sexual appetite appears in Sonnet 118, too: "Like as, to make our appetite more keen, / With eager compounds we our palate urge."
The comparison of the fair lord to a deity in the last 3 lines of this sonnet echoes the theme of Sonnets 105 and 108. "A god in love" clearly refers to the "older friend" of line 11, who is the fair lord. This idea is enforced by line 13, when the poet asks for welcome into the arms of the fair lord, which to him is the best thing beside heaven. However, the phrase "thy pure and most most loving breast" which ends the sonnet can be read as contradictory; though the breast is "pure," the double superlative "most most" can be read to mean that the fair lord himself has more lovers than anyone else.