I lifted my head to search his face, yet his features were faintvague in the fading light.

I like "vague" here because "search his face" gives me the feeling of the protagonist trying to make out the details, see what he's expressing, and "vague" gives that sense to me of seeing their face, but not being able to tell exactly what I'm looking for.

"If you ever say that again," I unintentionally spoke softlymuttered, my voice barely audible to even myself.,

I stole this from the other corrector. The use of "unintentional" here, I feel implies that the speaker is exerting some level of control over their voice and it comes out at the wrong volume, but it feels more like the line wants to imply that the speaker had no control over their voice's volume.

I replaced the period with a comma because the next quote really needs to be in the same sentence, I feel. These are not two disconnected thoughts, but one.

I'll take an action that will end our relationship forever."

"I'll take an action" feels a little formal and stiff for this kind of line. Especially for the emotional expression of the character. The previous line already contained "I will leave and never come back" so I would normally expect this line to have more emphasis and emotional intensity. I'm not sure what I might like to see here instead, though.

"You must understand," he spoke firmly, peering into my eyesPeering into my eyes, he spoke firmly. "You must understand... You can't keep dwelling on the past.

I felt that putting "Peering into my eyes, he spoke firmly." before the line would make his actions appear more deliberate, contemplative. He is exercising control over head to peer into your eyes, controlling his voice to speak firmly. That's in contrast to "'thing,' he said" lines which feel more like instant gut reactions coming immediately to mind and out the mouth.

It will kill you if you don't let it go—" ¶
¶
He stopped suddenly because I forcefully turned my head¶
¶
My head spun to the other side of the room, shutting my eyes and pleading for space.

"He stopped suddenly because I" felt slow and didn't match the pace of the action. I think we can imply the next action caused their pause by just stating the next action. But also, "I forcefully turned my head" feels intentional, but the image in my mind is of someone having a gut reaction to these words.

I'm suggesting just "My head spun" because speaking of the head as having acted on its own gives, to me, that feeling of a sudden gut impulse that caused a motion rather than an intentional, deliberate action.

I wouldn't have survived if it weren't for my hatewithout it.

He continued speaking softly and reasonably, yet I remained obstinately silent.

ditto what JoeTofu said.

A wave of hostile defensivenessindignation and painful incredulity crept into my heart, and suddenly everything inside me felt cold and tight.

Defensiveness is used to describe the behavior of someone who perceives they are attack when they are not and one will typically only admit they are acting defensively when breaking away from this thinking pattern.

I'm not really sure my use of "indignation" is correct here, but I get the sense of the pain of protagonist not feeling their pain is fully respected, realized, and accepted, and I think I would describe that as an indignity.

It was the taste of hatred—the hatred for him.

I'm almost wondering if this line could be made more impactful by making it a little terse.

> There was a taste, thick and bitter, at the back of my mouth. Hatred. For him.

I'm not sure.

Or perhaps it was merely (my) tears blurring my vision.

It would sound better without the first “my,” which is unnecessary.

"If you ever say that again," I [unintentionally spoke softly | muttered?], my voice barely audible to myself.

“Unintentionally spoke softly”seems weird. Which part was unintentional—the speaking or the softness?

"You must understand," he [spoke firmly | entreated?], peering into my eyes.

"You can't [keep dwelling] on the past.

“Dwelling” is a good, descriptive verb, but having a character actually speak words like this aloud makes them sound pretentious. If this is the impression you’re striving for, it’s fine.

It will kill you if you don't let it go—" ¶
¶
¶
He stopped suddenly because I [forcefully turned | twisted] my head to the other side of the room, shutting my eyes and pleading for space.

He continued speaking softly and [reasonably], yet I remained [obstinately] silent.

“Reasonably” and “obstinately” seem odd coming from a first-person narrator, because they imply that the narrator felt, in the moment, that the guy was being reasonable and that the narrator himself/herself was being obstinate. It doesn’t seem right that this character would judge these actions as reasonable or obstinate at that moment. “Hostile defensiveness and painful incredulity” in the next sentence seem similarly implausible to me—who analyzes their own feelings in this way, while they are feeling them?

It was the taste of hatred—(the) hatred for him.

The second “the” is unnatural, unless you are referring to a specific hatred for him that you already mentioned to the reader.

"It's over," I told myself, not even caring if I said those (these?) words out loud.

“Those” gives the impression that the narrator is standing at a distance from himself/herself, separated from what is happening at the moment. If you don’t want to create this impression, use “these.”

"I don't care if my hatred [will kill | kills] me or not.

More natural

I don't care (about) what will happen, as long as I can vanish."

“About” seems unnatural, unless the narrator is referring to a specific thing that he/she already knows will happen.