Bitter farewell

[dukealien]

Wonderful, on first read.  This POV has surely been novelized, but can't recall it as poetry until now.  T. H. White had some speculation on Guinevere's character, but always third-person omnisicient and not quite convincing.

Deserves (and will receive) a fuller critique, but on first reading a few points of grammar stuck out a bit:

It was not the queen that betrayed you,
it was the women,      Should be "woman," A. only  had one other (Morgana le Fey) and that's not the subject here anyway
but you betrayed me first,
left me to him,
then condemned me
when I sought him out,
but yours was the greater affair.

...

I can not do it Arthur, I am not you,   Period and reading seem to call for "cannot" here,  space reads as "am able to not do  it" rather than "am unable to do it"
I don't love this fickle land like you did.  "[A]s" rather than "like"
Though for love of you,
I will lay down with you mistress,  Should be "your," also, strictly, "lie" rather than "lay" unless you are going to be transitive ("lay myself down")
even though it will be my death.

Your choice of following the realistic rather than the Tennyson version (slow boat to Avalon) is fine - in that one their last meeting was at Glastonbury and A. was still alive.  Good navigation of the legend for purpose.