The action of falling is completed, and the speaker is describing something which had happened quite some time ago.
The use of first person voice and past tense conjugation is a literary device designed to make it read as if the character is narrating the story, so I am going to largely ignore the fact that, strictly speaking, everything is in the past. In terms of plot, gazing at Varien is "the present" here, and the description of speaker's feelings at this point reference events which happened at two different times in the past (or three, if you count the literary use of past tense).
A rough chronology of events:
- Varien was living with the heroine who loved him unrequitedly
- Varien changed the first time, finding his soulmate
- Varien left with his beloved
- The heroine's years of hopeless love fell like dead leaves (this is a single, completed action; she became crestfallen because at this point there's not even a tiny chance of being with him)
- The heroine started being sustained by knowing how much joy (that had filled him) Varien took in being with his beloved (this is a sustained state of action which occurred in the past)
(The last three items happen at the same time.)
- Varien changed again, losing the joy that had filled him when he met his soulmate
- The heroine met him again (extremely recent)
- The heroine gazed at Varien and realized how much he changed
- The joy which had been sustaining her stopped doing so (the previously initiated, ongoing action ceased)
That joy that had sustained me, knowing that he had found his soulmate at last, though all my years of hopeless love fell like dead leaves around my heart.
This sentence seems degenerate to me. All of that had sustained ... around my heart appears to be descriptive material and not contain a verb for the subject (that joy) to perform, making this a sentence fragment.
That had ... at last is an adjectival phrase describing that joy; it tells us which joy specifically. Let's remove it:
That joy, though all my years of hopeless love fell like dead leaves around my heart.
Clearly something's missing here; though what? But perhaps had been sustaining is the verb for that joy? If so, the second that needs to be removed:
That joy had sustained me, knowing that he had found his soulmate at last, though all my years of hopeless love fell like dead leaves around my heart.
But now there's a tense mismatch. The sustaining didn't start until Varien found his soulmate, which is the same time that the heroine's hopeless love fell. It cannot be the case that the sustaining starts or happens in the "present" because at that point Varien has changed, and the joy that had filled him when he left was gone. Additionally, the joy sustaining her is the same joy she saw when Varien left: The joy that had filled him... That joy.
Unfortunately, the next sentence doesn't make anything clearer; it just describes Varien's emotions in the "present":
He could barely speak for his anger and he was wild with helplessness.
I understand why there's an attempt to mix tenses here; the heroine is describing two past and intertwined events, one the initiation of a sustained action and the other a simple one time happening. They both occurred at the same time and have the same cause.
Here's how I'd fix the sentence:
That joy began sustaining me, knowing that he had found his soulmate at last, though all my years of hopeless love fell like dead leaves around my heart.
There's another, but extremely unlikely, possibility: typographical error on the author's part (the quote is accurate). Add an R (and also a comma for readability) and change though to through, making the meaning that joy sustained her during the years of hopeless love. In this case, those years necessarily continue up to the "present", when she sees the changes in Varien. Consider:
That joy that had sustained me, knowing that he had found his soulmate at last, through all my years of hopeless love, fell like dead leaves around my heart.
Now it's the joy which falls, not the years, and the falling occurs in the "present", not the distant past. It's still a completed action from a literary perspective, because this kind of first person narration necessarily means all the events took place in the past.
Regardless of whether you consider the time frame of gazing after Varien the present (plot perspective) or past (literary perspective), and even if you think it should be through rather than though, the falling is still a single event and not an ongoing state.