With mine faith & strength I held thy demons at
bay
With my faith and strength and by your side, I fought your demons
off to keep them away from you, but I also had to fight to keep
your demons away from me. Old english used to symbolise the
traditional role played by the girl here.. an old, idealistic type
of faith in love. Demons represent personal problems dealing with a
person's own mentality, and also external enemies / problems.
alas the wrong demons fought for yours did
sway
The man was fighting the wrong demons - ie. picking on the wrong
things to "fight" about in the relationship.
The man's faith & strength in the relationship buckled
In the end, the woman was also fighting the wrong demons because
the man turned out to be the demon that turned on the woman.
I never knew what this was all about- YOU?
This is the girl's reply to something the man told her.. It can be
read these ways :
I never knew what? (That) this was all about YOU?
(You said) I never knew what this was all about. (Was it all about)
YOU?
(You said) I never knew what this was all about.... (but did)
YOU?
And also these lines :
An empty vessel, cancerous, floating and
forlorn
Empty vessel - the man has nothing to give the girl because he
himself is an empty shell. All talk and no action ( "empty vessels
make the most noise" ).
cancerous - he seems to be like a part of the girl originally..
then later he turns into something ugly, a nightmare
floating - he has no direction
forlorn - representing his self-pity and his loneliness.. he likes
to see himself in this light and wants the girl to nurture him
(many girls have a thing for taking in "sorry-looking"
things)
For twas not love that you so seek....
But lust and gold that make men sick
The man gave up the relationship because he felt he needed not
love, but money (career) and women (plural).
You seek not to rid yourself of this plague
Remember the 1st few lines detailing how the man is always lonely?
Here it is compared with a sickness.. it's the man's "pattern", he
does not want to break out of his destructive cycle /
behaviour.
So go forth - seek - you'll burn just as I
said
The woman warned the man before that he would be the one who gets
hurt in the end if he only lived for money and women. She is
letting him go here : "ok if that's what you want go and find
it!"
I'll soar and burn with phoenix flames
The woman's telling the man that she will, like a phoenix, revive
(from the failed relationship) and burn brightly (turn beautiful,
find success in life). It also signifies change.
The serpent heart will burn resisting change
she is saying the cold-hearted man will suffer because he didn't
want to change for the better (as in, give up his destructive ways,
break out of his cycle).
I tried my best to combine the fewest words for maximum meaning.
Whew. I like expressing myself this way, I don't know why but I've
always liked literature 
