'Nevertheless you, O Sir Gauwaine, lie, Whatever may have happened through these years, God knows I speak the truth, saying that you lie.' (lines 46-8)
What Guenevere says here appears to be contradictory: she says that no matter what "may have happened" in the past, Sir Gauwaine's accusation is a "lie." How can that be true? Whose version of those past events is the "truth," and whose is a "lie"?
'[...] the sick Sure knowledge things would never be the same, However often Spring might be most thick Of blossoms and buds, smote on me, and I grew Careless of most things, let the clock tick, tick, To my unhappy pulse [...] (lines 71-76)
Guenevere realizes that the past cannot be repeated. Launcelot's presence at Arthur's court just reminds her that she is trapped in a loveless marriage. Time starts to work differently for her at this point: it flows past her ("tick, tick") without her being truly aware of it.
'While I was dizzied thus, old thoughts would crowd. Belonging to the time ere I was bought By Arthur's great name and his little love, [...]' (lines 81-83)
After Launcelot first arrives at Arthur's court, Guenevere is "dizzied" by her memories of the time before she was married – or, as she puts it, before she allowed herself to be "bought."
'Do I not know now of a day in Spring? No minute of that wild day ever slips From my memory: I hear thrushes sing, And wheresoever I may be, straightway Thoughts of it all come up with most fresh sting.' (lines 104-108)
No matter how much time has passed, Guenevere's memory of the "wild day" she kissed Launcelot in the garden stays "fresh." Just hearing the singing of "thrushes" is enough to bring her immediately back to that memory. You know how sometimes you associate things really strongly with a particular smell, taste, or song, so that when you hear that song or smell that scent you feel overwhelmed by a particular memory? That's what Guenevere is describing here.
'After that day why is it Guenevere grieves?' (line 141)
Guenevere asks herself why, with such an awesome memory to sustain her, she should ever be unhappy. She got what she wanted, didn't she? She got to smooch Launcelot in the garden. So why should she "grieve"? Of course, the prospect of being burnt at the stake by a bunch of angry knights might make anyone "grieve," no matter how many fond memories of garden smooching they have.
'Remember in what grave your mother sleeps' (line 153)
Guenevere tries to get Gauwaine's to take pity on her by calling up memories of his dead mother. She describes his mother's shameful death with some detail, but of course it doesn't work. (Check out the "Detailed Summary" if you're confused about this section, since Morris borrows from Arthurian legend without explaining the context.)