A summary of "Sonnet 35" must first focus on its intention expressed in lines 1 and 5: Be no longer "grieved" and everyone makes "faults." The poetic speaker's attempt to persuade the recipient to this intention is summed up in the sestet (9-14) but mostly in the ending couplet (13, 14) where he claims that he himself is an "accessary" to the recipient's fault, thereby (1) making himself equally guilty and (2) removing the recipient's guilt.
In the first quatrain of the octave (1-4), the speaker gives the reasons that the transgressor (the recipient of the sonnet) should not be grieved: everything in nature has beauty and faults too. The second quatrain (5-8) is built upon plays on words. He uses a complex string of word play to say that while all "men make faults," he also commits a fault (1) by excusing the other's error through a comparison to the duality of beautiful nature that has thorns as well a blossoms and (2) thereby causing himself to have a fault because (3) to thus forgive the other's fault is to (4) make that fault larger than it actually was; thus he ironically sins against the sinner.
The sestet brings in another string of word play beginning with "sensual" and "sense" wherein "sensual" refers to physical love (most likely withheld) and "sense" is rational reasoning. He says that by bringing reason to their quarrel, he (1) ceases being the other's adversary (the one wronged) and becomes the advocate (the defending lawyer) who, in a legal allusion, (2) files a complaint against himself (as the adversary), thus (3) causing a "civil war" of "love and hate" (he is the one wronged and he is the advocate of the other's innocence both at the same time) and in thus accusing himself before the court, which is being alluded to, makes himself an accessory to the wrong committed against himself: an "accessary" "To that sweet thief which sourly robs [love] from me."