One of the main themes of my book is death and sacrifice. There's a lot of necromancy, ghosts, etc. The main character has already been brought back to life once before without her consent, which was at the start of the story. I was thinking of concluding the story with her dying in a final act of self sacrifice to kind of bookend the whole narrative and tie into the themes of self sacrifice and death. Problem is I love her too much, I'm too attached to her and I'm too invested in her romantic subplot. Do I bite the bullet and just do it?
Possibly, it depends on your overall themes of the story. There's a general trend in fiction for undead characters to be killed off by the end of a particular novel or series, but that's less because of the nature of the characters and more because it's hard to come up with a satisfying happy ending for them and the state of undeath is kind of...awkward. So the characters either get turned back to human or killed off under the assumption that the afterlife is a "better place" (it is frequently never shown to be that way. It's more of a way to awkwardly shuffle the undead character off-stage to avoid discussing the thorny moral implications of someone coming back from the dead.
If self-sacrifice is a major theme of your plot, you have a good reason for killing her off. This doesn't mean you have to, only that it is an acceptable outcome looking at the narrative from a solely thematic perspective. This is also only from the details you have given, associated details may make it more problematic.
It also depends on the setting. One major issue with any kind of resurrection in media is it erodes verisimillitude (because we can't resurrect the dead in real life) and raises the question of "if you can bring back one person, why morally shouldn't we bring back all of them". But if necromancy is already a part of the setting that is going to affect the tone of the story. That said, using a resurrection as an inciting incident is a good "one-time use get out of jail free card" because your readers expect something weird to trigger your plot. The Courier's miraculous near-miss with death at the beginning of Fallout: New Vegas is a good example of this. Resurrection in this case is handled like a miraculous second chance that isn't going to get repeated. If they get fully resurrected as the inciting incident there are no expectations for the hero to die (though having a heroic sacrifice at the end of the story won't be unexpected by the audience in general).
The big issue I see is unless you play it very carefully, your character's death comes off less like a heroic sacrifice and more like a subconscious desire to commit suicide. Especially if they've been brought back once before. See: Buffy the Vampire Slayer. A heroic sacrifice is about a character valuing someone or something else more than they value their own life. They still want to live, they just don't see saving both as an option. Dying because they want to die and saving someone else as a bonus isn't a sacrifice. An exception might be made for "they were dying of a terminal illness anyway", but that's become recently criticized as a cop-out and a cliche for writers to blunt the senselessness of a character's death because "oh, they were dying anyway".
It also depends on the other themes of your novel. I had an issue similar to yours once where I had a character that was expected to die but I couldn't bring myself to do it. It was only after a great deal of introspection that I realized the reason I couldn't do it was that the character's death conflicted with numerous other subplots in the novel and their death left the readers with a great sense of unease and despair rather than one of catharsis. Having them die in that manner would have violated the themes of the novel and made the message of the story hypocritical. Yours may be a similar issue.
Write what makes you happy as a writer. Maybe that does mean killing off a character for the sake of your long-term happiness, but don't include something simply because you feel you have to.