Meter is built on syllables.
As a specific example, let us turn to the first stanza.
In lands of green and virtues true,
Where gardens sprawl and heroes too,
There always was to be a blight,
Challenging a "heroic" might.
. . . . . . . . (8 syllables)
. . . . . . . . (8 syl)
. . . . . . . . (8 syl)
. . . . . . . . (8 syl)
The meter of the first stanza is a steady 8. While not all poems have to be straight in meter, this stanza sets precedence for straight 8s.
When in battle rode sword and shield,
Adorned with serpents and charging field,
Fleeing foes struck by battle woes,
Serpents striking at sun-low.¹
. . . . . . . . (8 syl)
. . . . . . . . . (9 syl)
. . . . . . . . (8 syl)
. . . . . . . (7 syl)
Here you deviate from the pattern, with the lines of 9 and 7 being especially clunky due to this. Both stanzas are notated as Verse 1. In a typical epic, the stanzas in any one verse would be consistent with one another.
By dawn the rumour it had spread,
Of serpent venom and thousands dead,
Mythical creatures made by God,
Struck heroes down as if but a nod.²
. . . . . . . . (8 syl)
. . . . . . . . . (9 syl)
. . . . . . . . (8 syl)
. . . . . . . . (8 syl)
Here you have one line with an extra syllable.
Kings sent their men to hunt them down,
With soldiers burning town on town,
To no avail they searched 'til light,
Much to the serpent's right-hands delight.
. . . . . . . . (8 syl)
. . . . . . . . (8 syl)
. . . . . . . . (8 syl)
. . . . . . . . . (9 syl)
Another line with an extra syllable.
To avenge the people ruthlessly killed,
The Serpents set out over the hills,
Slaughtering soldier and causing fray,
Towers burning, stuffed with hay.
. . . . . . . . . . (10 syl)
. . . . . . . . . (9 syl)
. . . . . . . . . (9 syl)
. . . . . . . (7 syl)
Woah there.
To that day the people sing,
"That day they saved us from our King!"
And forever cemented in history,
The hands of the serpent remains a mystery.
. . . . . . . (7 syl)
. . . . . . . . (8 syl)
. . . . . . . . . . . (11 syl)
. . . . . . . . . . . . (12 syl)
Uh oh.
Here's what I'd recommend to fix your meter problem: using apostrophes to conceal certain syllables. This is called an elision (I pray to the grammar gods I'm using that word correctly). for example: "And for'ere cemented in hist'ry." would bring the line down to 9 syllables instead of the uncomely 11. Read it a few times out loud in comparison, counting syllables. Elisions take practice, and they don't apply to all words.
Overall, I'd say you really should pick a steady meter for each verse (or at least a regularly alternating one: i.e. 8 7 8 7).
Rhythm in poetry is something completely different that would take quite a hot second to explain. It is usually inherent in straight metered poems though, so you shouldn't have to worry too much about it(?).